Well, it seems to be exactly like type classes. What if instead of implementing this "roles" we simply add a type class, say, "HasRepresentationalArgument", which can be (and is) derived automatically. Of course, multiple arguments could be a problem, but, since we already have polymorphic kinds, not a big one. Отправлено с iPad

08 окт. 2013 г., в 0:16, Richard Eisenberg написал(а):

You raise an excellent point, and yes, your understanding of how roles work in this case is correct.

The problem with your proposal is that it's rather involved to implement and maintain -- essentially, every type and type variable would need to be annotated with both a role and a kind. (These annotations would generally be invisible to users, but quite apparent to implementors.) The current solution is admittedly incomplete in this area, but it requires tracking roles only for parameters to datatypes and classes. We're waiting to see how important this particular issue is in practice before committing to implementing it (a medium-sized project) and maintaining it into perpetuity.

Richard

...

On Oct 7, 2013, at 3:55 PM, migmit wrote:

Something bugs me here.

If some type variable a is used as a parameter to another type variable t, then it's considered nominal. I suppose, that's because it is possible that it would be nominal for some specific t. But we might just know that in our application it's always representational, for every possible t that we would ever use. In this case, we might want to a) explicitly state that t's type parameter should always be representational, and b) at the same time make a representational. Seems like a probable scenario to me.

Отправлено с iPad

...

07 окт. 2013 г., в 17:26, Richard Eisenberg написал(а):

As you may have heard, /roles/ will be introduced with GHC 7.8. Roles are a mechanism to allow for safe 0-cost conversions between newtypes and their base types. GeneralizedNewtypeDeriving (GND) already did this for class instances, but in an unsafe way -- the feature has essentially been retrofitted to work with roles. This means that some uses of GND that appear to be unsafe will no longer work. See the wiki page [1] or slides from a recent presentation [2] for more info.

[1] : http://ghc.haskell.org/trac/ghc/wiki/Roles [2] : http://www.cis.upenn.edu/~eir/papers/2013/roles/roles-slides.pdf

I am writing because it's unclear what the *default* role should be -- that is, should GND be allowed by default? Examples follow, but the critical issue is this:

* If we allow GND by default anywhere it is type-safe, datatypes (even those that don't export constructors) will not be abstract by default. Library writers would have to use a role annotation everywhere they wish to declare a datatype they do not want users to be able to inspect. (Roles still keep type-*un*safe GND from happening.)

* If we disallow GND by default, then perhaps lots of current uses of GND will break. Library writers will have to explicitly declare when they wish to permit GND involving a datatype.

Which do we think is better?

Examples: The chief example demonstrating the problem is (a hypothetical implementation of) Set:

...

module Set (Set) where -- note: no constructors exported!

data Set a = MkSet [a] insert :: Ord a => a -> Set a -> Set a ...

...

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving #-} module Client where

import Set

newtype Age = MkAge Int deriving Eq

instance Ord Age where (MkAge a) `compare` (MkAge b) = b `compare` a -- flip operands, reversing the order

class HasSet a where getSet :: Set a

instance HasSet Int where getSet = insert 2 (insert 5 empty)

deriving instance HasSet Age

good :: Set Int good = getSet

bad :: Set Age bad = getSet

According to the way GND works, `good` and `bad` will have the same runtime representation. But, using Set operations on `bad` would indeed be bad -- because the Ord instance for Age is different than that for Int, Set operations will fail unexpectedly on `bad`. The problem is that Set should really be abstract, but we've been able to break this abstraction with GND. Note that there is no type error in these operations, just wrong behavior.

So, if we default to *no* GND, then the "deriving" line above would have an error and this problem wouldn't happen. If we default to *allowing* GND, then the writer of Set would have to include

...

type role Set nominal in the definition of the Set module to prevent the use of GND. (Why that peculiar annotation? See the linked further reading, above.)

Although it doesn't figure in this example, a library writer who wishes to allow GND in the default-no scenario would need a similar annotation

...

type role Foo representational to allow it.

There are clearly reasons for and against either decision, but which is better? Let the users decide!

Discussion time: 2 weeks.

Thanks! Richard _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

We considered this for a while, but it led to a strange design -- to do it right, you would have to import all constructors for all datatypes *recursively* out to the leaves, starting at the datatypes mentioned in the class for which you wanted to use GND. This would mean potentially a whole lot of imports for symbols not actually used in the text of a program. Richard On Oct 7, 2013, at 4:33 PM, Ganesh Sittampalam wrote:

Is it possible to tie the role to whether the data constructor is visible or not?

On 07/10/2013 14:26, Richard Eisenberg wrote:

...

As you may have heard, /roles/ will be introduced with GHC 7.8. Roles are a mechanism to allow for safe 0-cost conversions between newtypes and their base types. GeneralizedNewtypeDeriving (GND) already did this for class instances, but in an unsafe way -- the feature has essentially been retrofitted to work with roles. This means that some uses of GND that appear to be unsafe will no longer work. See the wiki page [1] or slides from a recent presentation [2] for more info.

[1] : http://ghc.haskell.org/trac/ghc/wiki/Roles [2] : http://www.cis.upenn.edu/~eir/papers/2013/roles/roles-slides.pdf

I am writing because it's unclear what the *default* role should be -- that is, should GND be allowed by default? Examples follow, but the critical issue is this:

* If we allow GND by default anywhere it is type-safe, datatypes (even those that don't export constructors) will not be abstract by default. Library writers would have to use a role annotation everywhere they wish to declare a datatype they do not want users to be able to inspect. (Roles still keep type-*un*safe GND from happening.)

* If we disallow GND by default, then perhaps lots of current uses of GND will break. Library writers will have to explicitly declare when they wish to permit GND involving a datatype.

Which do we think is better?

Examples: The chief example demonstrating the problem is (a hypothetical implementation of) Set:

...

module Set (Set) where -- note: no constructors exported!

data Set a = MkSet [a] insert :: Ord a => a -> Set a -> Set a ...

...

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving #-} module Client where

import Set

newtype Age = MkAge Int deriving Eq

instance Ord Age where (MkAge a) `compare` (MkAge b) = b `compare` a -- flip operands, reversing the order

class HasSet a where getSet :: Set a

instance HasSet Int where getSet = insert 2 (insert 5 empty)

deriving instance HasSet Age

good :: Set Int good = getSet

bad :: Set Age bad = getSet

According to the way GND works, `good` and `bad` will have the same runtime representation. But, using Set operations on `bad` would indeed be bad -- because the Ord instance for Age is different than that for Int, Set operations will fail unexpectedly on `bad`. The problem is that Set should really be abstract, but we've been able to break this abstraction with GND. Note that there is no type error in these operations, just wrong behavior.

...

type role Set nominal in the definition of the Set module to prevent the use of GND. (Why that

So, if we default to *no* GND, then the "deriving" line above would have an error and this problem wouldn't happen. If we default to *allowing* GND, then the writer of Set would have to include peculiar annotation? See the linked further reading, above.)

Although it doesn't figure in this example, a library writer who wishes to allow GND in the default-no scenario would need a similar annotation

...

type role Foo representational to allow it.

There are clearly reasons for and against either decision, but which is better? Let the users decide!

Discussion time: 2 weeks.

Thanks! Richard

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Pedro is suggesting a way for a Haskell type-level program to gain access to role information. This might indeed be useful, but it doesn't seem terribly related to the problem of defaults / abstraction. The problem has to do with definitions like these:

module A where data S a b = S1 a | S2 b data T a b = MkT (S a b)

module B where import A ( {- what goes here? -} )

class C a where mkT :: T Bool a

instance C Int where ... newtype Age = MkAge Int deriving C

What constructors do we need in order to convert the (C Int) instance to (C Age) by hand? To me, it looks like we need MkT and S2, but not S1. Yet, this is not obvious and seems to be quite confusing. I hope this helps understanding the issue! Richard On Oct 8, 2013, at 4:01 AM, José Pedro Magalhães wrote:

Hi,

On Tue, Oct 8, 2013 at 3:21 AM, Richard Eisenberg wrote: We considered this for a while, but it led to a strange design -- to do it right, you would have to import all constructors for all datatypes *recursively* out to the leaves, starting at the datatypes mentioned in the class for which you wanted to use GND. This would mean potentially a whole lot of imports for symbols not actually used in the text of a program.

I'm not sure I understand why constructors are involved in this. Wouldn't something like the following potentially be useful?

data Role = Nominal | Representational | Phantom | Fun Role Role

type family HasRole (t :: k) :: Role

data MyData a b = MyData a data MyGADT a b where MyGADT :: MyGADT a Int

type instance HasRole MyData = Fun Representational Phantom type instance HasRole MyGADT = Fun Representational Nominal type instance HasRole Traversable = Nominal

HasRole instances would be automatically given by GHC.

Cheers, Pedro

I don't understand it either. Type family solution, however, seems wrong. See, if we, somehow, make something nominal when it has to be representational — well, some code won't compile, but nothing really bad happens. If, on the other hand, we by some miracle make something representational when in should be nominal — we can get a runtime error. It seems to be very similar to how type classes work, with "nominal" being the default, and "representational" a type class. Consider, for example, the "Tricky" example from the slides, slightly changed: data Tricky2 a b c = MkTricky2 (a c) (b c) Currently parameter c would be nominal. I suggest that it should be representational if and only if it's representational for BOTH a and b. WIth type classes it would be very simple: instance (HasRepresentationalParameter a, HasRepresentationalParameter b) => HasRepresentationalParameter (Tricky2 a b) With type families... well, apparently I don't have enough milliolegs to figure out how to do it. On Tue, Oct 8, 2013 at 12:01 PM, José Pedro Magalhães wrote:

Hi,

On Tue, Oct 8, 2013 at 3:21 AM, Richard Eisenberg wrote:

...

We considered this for a while, but it led to a strange design -- to do it right, you would have to import all constructors for all datatypes *recursively* out to the leaves, starting at the datatypes mentioned in the class for which you wanted to use GND. This would mean potentially a whole lot of imports for symbols not actually used in the text of a program.

I'm not sure I understand why constructors are involved in this. Wouldn't something like the following potentially be useful?

data Role = Nominal | Representational | Phantom | Fun Role Role

type family HasRole (t :: k) :: Role

data MyData a b = MyData a data MyGADT a b where MyGADT :: MyGADT a Int

type instance HasRole MyData = Fun Representational Phantom type instance HasRole MyGADT = Fun Representational Nominal type instance HasRole Traversable = Nominal

HasRole instances would be automatically given by GHC.

Cheers, Pedro

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

I don't quite agree with your analysis, Edward. Eq can be auto-derived, so it makes for a confusing example. Let's replace Eq in your example with this class:

class C a where c_meth :: a -> a -> Bool

Then, your example leads to the same embarrassing state of affairs: coercing a dictionary for (C Int) to one for (C Bar). But, I would argue that we still want C's parameter to have a representational role. Why? Consider this:

data Blargh = ... instance C Blargh where ...

newtype Baz = MkBaz Blargh deriving C

We want that last line to work, using GeneralizedNewtypeDeriving. This hinges on C's parameter's role being representational. I think that what you've witnessed is a case of bug #8338 (http://ghc.haskell.org/trac/ghc/ticket/8338). This is a problem, in my view, and it seems to touch on roles, but I'm not completely sure of their relationship. So, I think that classes should keep their representational roles (regardless of the decision on datatypes -- Haskell doesn't really support "abstract" classes), but perhaps we have to find a way to stop these incoherent instances from forming. Maybe the use of a constraint makes a datatype's role be nominal? Richard On Oct 9, 2013, at 1:55 PM, Edward Kmett wrote:

I just noticed there is a pretty big issue with the current default role where typeclasses are concerned!

When implementing Data.Type.Coercion I had to use the fact that I could apply coerce to the arguments of

data Coercion a b where Coercion :: Coercible a b => Coercion a b

This makes sense as Coercion itself has two representational arguments.

This struck me as quite clever, so I went to test it further.

data Foo a where Foo :: Eq a => Foo a

newtype Bar = Bar Int instance Eq Bar where _ == _ = False

I fully expected the following to fail:

coerce (Foo :: Foo Int) :: Foo Bar

but instead it succeeded.

This means I was able to convert a dictionary Eq Int into a dictionary for Eq Bar!

This indicates that Eq (actually all) of the typeclasses are currently marked as having representational, when actually it strikes me that (almost?) none of them should be.

Coercible is the only case I can think of in base of a class with two representational arguments, but this is only valid because we prevent users from defining Coercible instances manually.

If I try again with a new typeclass that has an explicit nominal role

type role Eq nominal class Eq a instance Eq Int instance Eq Bar

then I get the failure to derive Coercible (Foo Int) (Foo Bar) that I'd expect.

This indicates two big issues to me:

1.) At the very least the default role for type classes should be nominal for each argument. The very point of an instance is to make a nominal distinction after all. =)

2.) It also indicates that making any typeclass with a representational (/ phantom?) argument shouldn't be possible in valid SafeHaskell, as you can use it to subvert the current restrictions on OverlappingInstances.

-Edward

On Wed, Oct 9, 2013 at 12:07 PM, Iavor Diatchki wrote: Hello,

My preference would be for the following design:

1. The default datatypes for roles are Nominal, but programmers can add annotations to relax this. 2. Generlized newtype deriving works as follows: we can coerce a dictionary for `C R` into `C T`, as long as we can coerce the types of all methods instantiated with `R`, into the corresponding types instantiated with `T`. In other words, we are pretending that we are implementing all methods by using `coerce`.

As far as I can see this safe, and matches what I'd expect as a programmer. It also solves the problem with the `Set` example: because `Set` has a nominal parameter, we cannot coerce `Set Int` into `Set MyAge` and, hence, we cannot derive an instance of `MyAge` for `HasSet`. An added benefit of this approach is that when newtype deriving fails, we can give a nicer error saying exactly which method causes the problem.

-Iavor

On Mon, Oct 7, 2013 at 6:26 AM, Richard Eisenberg wrote: As you may have heard, /roles/ will be introduced with GHC 7.8. Roles are a mechanism to allow for safe 0-cost conversions between newtypes and their base types. GeneralizedNewtypeDeriving (GND) already did this for class instances, but in an unsafe way -- the feature has essentially been retrofitted to work with roles. This means that some uses of GND that appear to be unsafe will no longer work. See the wiki page [1] or slides from a recent presentation [2] for more info.

[1] : http://ghc.haskell.org/trac/ghc/wiki/Roles [2] : http://www.cis.upenn.edu/~eir/papers/2013/roles/roles-slides.pdf

I am writing because it's unclear what the *default* role should be -- that is, should GND be allowed by default? Examples follow, but the critical issue is this:

* If we allow GND by default anywhere it is type-safe, datatypes (even those that don't export constructors) will not be abstract by default. Library writers would have to use a role annotation everywhere they wish to declare a datatype they do not want users to be able to inspect. (Roles still keep type-*un*safe GND from happening.)

* If we disallow GND by default, then perhaps lots of current uses of GND will break. Library writers will have to explicitly declare when they wish to permit GND involving a datatype.

Which do we think is better?

Examples: The chief example demonstrating the problem is (a hypothetical implementation of) Set:

...

module Set (Set) where -- note: no constructors exported!

data Set a = MkSet [a] insert :: Ord a => a -> Set a -> Set a ...

...

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving #-} module Client where

import Set

newtype Age = MkAge Int deriving Eq

instance Ord Age where (MkAge a) `compare` (MkAge b) = b `compare` a -- flip operands, reversing the order

class HasSet a where getSet :: Set a

instance HasSet Int where getSet = insert 2 (insert 5 empty)

deriving instance HasSet Age

good :: Set Int good = getSet

bad :: Set Age bad = getSet

According to the way GND works, `good` and `bad` will have the same runtime representation. But, using Set operations on `bad` would indeed be bad -- because the Ord instance for Age is different than that for Int, Set operations will fail unexpectedly on `bad`. The problem is that Set should really be abstract, but we've been able to break this abstraction with GND. Note that there is no type error in these operations, just wrong behavior.

So, if we default to *no* GND, then the "deriving" line above would have an error and this problem wouldn't happen. If we default to *allowing* GND, then the writer of Set would have to include

...

type role Set nominal in the definition of the Set module to prevent the use of GND. (Why that peculiar annotation? See the linked further reading, above.)

Although it doesn't figure in this example, a library writer who wishes to allow GND in the default-no scenario would need a similar annotation

...

type role Foo representational to allow it.

There are clearly reasons for and against either decision, but which is better? Let the users decide!

Discussion time: 2 weeks.

Thanks! Richard

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Now I think we're on the same page, and I *am* a little worried about the sky falling because of this. (That's not a euphemism -- I'm only a little worried.) Well, maybe I should be more worried. The whole idea of roles is to protect against type-unsoundness. They are doing a great job of that here -- no problem that we've discussed in this thread is a threat against type safety. The issue immediately at hand is about coherence (or perhaps you call it confluence) of instances. Roles do not address the issue of coherence at all, and thus they fail to protect against coherence attacks. It would take More Thought to reformulate roles (or devise something else) to handle coherence. It's worth pointing out that this isn't a new problem, exactly. Bug #8338 shows a way to produce incoherence using only the GADTs extension. (It does need 4 modules, though.) I conjecture that incoherence is also possible through GeneralizedNewtypeDeriving, both as it existed in GHC 7.6.3 and in 7.8, so it's not an issue with Coercible, exactly. It's just that Coercible allows you to get incoherence with so much less fuss than before! Wait! I have an idea! The way I've been describing GND all along has been an abbreviation. GHC does not coerce a dictionary from, say, Ord Int to Ord Age. Instead, GHC mints a fresh dictionary for Ord Age where all the methods are implemented as coerced versions of the methods for Ord Int. (I'm not sure why it's implemented this way, which is why I've elided this detail in just about every conversation on the topic.) With this in mind, I have a proposal: 1) All parameters of all classes have nominal role. 2) Classes also store one extra bit per parameter, saying whether all uses of that parameter are representational. Essentially, this bit says whether that parameter is suitable for GND. (Currently, we could just store for the last parameter, but we can imagine extensions to the GND mechanism for other parameters.) Because GND is implemented using coercions on each piece instead of wholesale, the nominal roles on classes won't get in the way of proper use of GND. An experiment (see below for details) also confirms that even superclasses work well with this idea -- the superclasses aren't coerced. Under this proposal, dictionaries can never be coerced, but GND would still seem to work. Thoughts? Richard Experiment:

newtype Age = MkAge Int

instance Eq Age where _ == _ = False

deriving instance Ord Age

useOrdInstance :: Ord a => a -> Bool useOrdInstance x = (x == x)

What does `useOrdInstance (MkAge 5)` yield? It yields `False` (in HEAD). This means that the existing GND mechanism (I didn't change anything around this part of the code) uses superclass instances for the *newtype*, not for the *base type*. So, even with superclasses, class dictionaries don't need to be coerced. On Oct 9, 2013, at 2:52 PM, Edward Kmett wrote:

I'd be happy to be wrong. =)

We do seem to have stumbled into a design paradox though.

To make it so you can use roles in GeneralizedNewtypeDeriving hinges on the parameter's role being representational, but making it representational means users can also use coerce to turn dictionaries into other dictionaries outside of GND.

This is quite insidious, as another dictionary for Eq or Ord may exist for that type, where it becomes unsound as the generated dictionary may be used to destroy confluence.

This means that even if something like Set has a nominal argument it isn't safe, because you can attack the invariants of the structure via Ord.

newtype Bad = Bad Int deriving Eq instance Ord Bad where compare (Bad a) (Bad b) = compare b a

If Ord has a representational role then I can use coerce to convert a dictonary Ord Bad to Ord Int, then work locally in a context where that is the dictionary for Ord Int that I get when I go to do an insert or lookup.

I don't mean to sound like the sky is falling, but I do worry that the 'use of a constraint in a data type' may not be necessary or sufficient. That is a lot of surface area to defend against attack.

I am not sure that I actually need a data type to coerce a dictionary. It seems likely that I could do it with just a well crafted function argument and ScopedTypeVariables, but my version of HEAD is a bit too mangled at the moment to give it a try.

-Edward

On Wed, Oct 9, 2013 at 2:09 PM, Richard Eisenberg wrote: I don't quite agree with your analysis, Edward.

Eq can be auto-derived, so it makes for a confusing example. Let's replace Eq in your example with this class:

...

class C a where c_meth :: a -> a -> Bool

Then, your example leads to the same embarrassing state of affairs: coercing a dictionary for (C Int) to one for (C Bar).

But, I would argue that we still want C's parameter to have a representational role. Why? Consider this:

...

data Blargh = ... instance C Blargh where ...

newtype Baz = MkBaz Blargh deriving C

We want that last line to work, using GeneralizedNewtypeDeriving. This hinges on C's parameter's role being representational.

I think that what you've witnessed is a case of bug #8338 (http://ghc.haskell.org/trac/ghc/ticket/8338). This is a problem, in my view, and it seems to touch on roles, but I'm not completely sure of their relationship.

So, I think that classes should keep their representational roles (regardless of the decision on datatypes -- Haskell doesn't really support "abstract" classes), but perhaps we have to find a way to stop these incoherent instances from forming. Maybe the use of a constraint makes a datatype's role be nominal?

Richard

On Oct 9, 2013, at 1:55 PM, Edward Kmett wrote:

...

I just noticed there is a pretty big issue with the current default role where typeclasses are concerned!

When implementing Data.Type.Coercion I had to use the fact that I could apply coerce to the arguments of

data Coercion a b where Coercion :: Coercible a b => Coercion a b

This makes sense as Coercion itself has two representational arguments.

This struck me as quite clever, so I went to test it further.

data Foo a where Foo :: Eq a => Foo a

newtype Bar = Bar Int instance Eq Bar where _ == _ = False

I fully expected the following to fail:

coerce (Foo :: Foo Int) :: Foo Bar

but instead it succeeded.

This means I was able to convert a dictionary Eq Int into a dictionary for Eq Bar!

This indicates that Eq (actually all) of the typeclasses are currently marked as having representational, when actually it strikes me that (almost?) none of them should be.

Coercible is the only case I can think of in base of a class with two representational arguments, but this is only valid because we prevent users from defining Coercible instances manually.

If I try again with a new typeclass that has an explicit nominal role

type role Eq nominal class Eq a instance Eq Int instance Eq Bar

then I get the failure to derive Coercible (Foo Int) (Foo Bar) that I'd expect.

This indicates two big issues to me:

1.) At the very least the default role for type classes should be nominal for each argument. The very point of an instance is to make a nominal distinction after all. =)

2.) It also indicates that making any typeclass with a representational (/ phantom?) argument shouldn't be possible in valid SafeHaskell, as you can use it to subvert the current restrictions on OverlappingInstances.

-Edward

On Wed, Oct 9, 2013 at 12:07 PM, Iavor Diatchki wrote: Hello,

My preference would be for the following design:

1. The default datatypes for roles are Nominal, but programmers can add annotations to relax this. 2. Generlized newtype deriving works as follows: we can coerce a dictionary for `C R` into `C T`, as long as we can coerce the types of all methods instantiated with `R`, into the corresponding types instantiated with `T`. In other words, we are pretending that we are implementing all methods by using `coerce`.

As far as I can see this safe, and matches what I'd expect as a programmer. It also solves the problem with the `Set` example: because `Set` has a nominal parameter, we cannot coerce `Set Int` into `Set MyAge` and, hence, we cannot derive an instance of `MyAge` for `HasSet`. An added benefit of this approach is that when newtype deriving fails, we can give a nicer error saying exactly which method causes the problem.

-Iavor

On Mon, Oct 7, 2013 at 6:26 AM, Richard Eisenberg wrote: As you may have heard, /roles/ will be introduced with GHC 7.8. Roles are a mechanism to allow for safe 0-cost conversions between newtypes and their base types. GeneralizedNewtypeDeriving (GND) already did this for class instances, but in an unsafe way -- the feature has essentially been retrofitted to work with roles. This means that some uses of GND that appear to be unsafe will no longer work. See the wiki page [1] or slides from a recent presentation [2] for more info.

[1] : http://ghc.haskell.org/trac/ghc/wiki/Roles [2] : http://www.cis.upenn.edu/~eir/papers/2013/roles/roles-slides.pdf

I am writing because it's unclear what the *default* role should be -- that is, should GND be allowed by default? Examples follow, but the critical issue is this:

* If we allow GND by default anywhere it is type-safe, datatypes (even those that don't export constructors) will not be abstract by default. Library writers would have to use a role annotation everywhere they wish to declare a datatype they do not want users to be able to inspect. (Roles still keep type-*un*safe GND from happening.)

* If we disallow GND by default, then perhaps lots of current uses of GND will break. Library writers will have to explicitly declare when they wish to permit GND involving a datatype.

Which do we think is better?

Examples: The chief example demonstrating the problem is (a hypothetical implementation of) Set:

...

module Set (Set) where -- note: no constructors exported!

data Set a = MkSet [a] insert :: Ord a => a -> Set a -> Set a ...

...

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving #-} module Client where

import Set

newtype Age = MkAge Int deriving Eq

instance Ord Age where (MkAge a) `compare` (MkAge b) = b `compare` a -- flip operands, reversing the order

class HasSet a where getSet :: Set a

instance HasSet Int where getSet = insert 2 (insert 5 empty)

deriving instance HasSet Age

good :: Set Int good = getSet

bad :: Set Age bad = getSet

According to the way GND works, `good` and `bad` will have the same runtime representation. But, using Set operations on `bad` would indeed be bad -- because the Ord instance for Age is different than that for Int, Set operations will fail unexpectedly on `bad`. The problem is that Set should really be abstract, but we've been able to break this abstraction with GND. Note that there is no type error in these operations, just wrong behavior.

So, if we default to *no* GND, then the "deriving" line above would have an error and this problem wouldn't happen. If we default to *allowing* GND, then the writer of Set would have to include

...

type role Set nominal in the definition of the Set module to prevent the use of GND. (Why that peculiar annotation? See the linked further reading, above.)

Although it doesn't figure in this example, a library writer who wishes to allow GND in the default-no scenario would need a similar annotation

...

type role Foo representational to allow it.

There are clearly reasons for and against either decision, but which is better? Let the users decide!

Discussion time: 2 weeks.

Thanks! Richard

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On Wed, Oct 9, 2013 at 3:21 PM, Richard Eisenberg wrote:

Now I think we're on the same page, and I *am* a little worried about the sky falling because of this. (That's not a euphemism -- I'm only a little worried.)

=)

Wait! I have an idea! The way I've been describing GND all along has been an abbreviation. GHC does not coerce a dictionary from, say, Ord Int to Ord Age. Instead, GHC mints a fresh dictionary for Ord Age where all the methods are implemented as coerced versions of the methods for Ord Int. (I'm not sure why it's implemented this way, which is why I've elided this detail in just about every conversation on the topic.) With this in mind, I have a proposal:

1) All parameters of all classes have nominal role. 2) Classes also store one extra bit per parameter, saying whether all uses of that parameter are representational. Essentially, this bit says whether that parameter is suitable for GND. (Currently, we could just store for the last parameter, but we can imagine extensions to the GND mechanism for other parameters.)

Because GND is implemented using coercions on each piece instead of wholesale, the nominal roles on classes won't gehingt in the way of proper use of GND. An experiment (see below for details) also confirms that even superclasses work well with this idea -- the superclasses aren't coerced.

Under this proposal, dictionaries can never be coerced, but GND would still seem to work.

Thoughts?

This strikes me as a remarkably straightforward solution. Does it strike you as something implementable in time for 7.8 though?

Richard

Experiment:

newtype Age = MkAge Int

instance Eq Age where _ == _ = False

deriving instance Ord Age

useOrdInstance :: Ord a => a -> Bool useOrdInstance x = (x == x)

What does `useOrdInstance (MkAge 5)` yield? It yields `False` (in HEAD). This means that the existing GND mechanism (I didn't change anything around this part of the code) uses superclass instances for the *newtype*, not for the *base type*. So, even with superclasses, class dictionaries don't need to be coerced.

Upon reflection it makes a lot of sense that GND has to mint a new dictionary, because the superclasses may differ, like you showed here. -Edward

Hi, not sure if this is not old news to you all, but I think that for this discussion, it helps to consider these two aspects of a class instance separately: (1) An instance is a record of functions (2) An instance is a function of sorts¹ from types to (1) and clearly, type parameters of (1) can be representational, but the function in (2) should have its parameters nominal. Therefore it is fine to coerce the dictionary of a function (and would we want to implement GND this ways, that would be fine), but not a type involving a constraint. Inside GHC, as far as I can tell, (2) exists in the form of the instance metadata, and disappears after desugaring, while (1) is the actual dictionary that exists in core as a regular data type. So the conclusion is indeed: Let type class constraints have a nominal role, and all is fine. Greetings, Joachim ¹ well, a kind of function. But not that type of kind, but the other type. Sort of, at least. -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org

Hi, Am Mittwoch, den 09.10.2013, 23:18 -0400 schrieb Richard Eisenberg:

On Oct 9, 2013, at 6:24 PM, Joachim Breitner wrote:

...

So the conclusion is indeed: Let type class constraints have a nominal role, and all is fine.

But, then it would seem that any class with a superclass wouldn't be compatible with GND. Do you see that detail as a consequence of this design?

I think this approach might work, but I'm not yet convinced.

given that we coerce the fields individually already, and are not going to change that, I don’t think there is a problem with superclasses. Even more so: The instance datatype of the subclass will have a field that contains the instance _datatype_ of the superclass, not a field with a type class constraint (because as soon as we talk about dictionaries, we are in Core, where the instance _type functions_ have already been resolved), which would be representational. It probably is confusing that (IIRC) the same TyCon is used for both uses of classes: At the Haskell level, as a function on types; at the core level, as a regular datatype. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org

The way I've been describing GND all along has been an abbreviation. GHC does not coerce a dictionary from, say, Ord Int to Ord Age. Instead, GHC mints a fresh dictionary for Ord Age where all the methods are implemented as coerced versions of the methods for Ord Int. (I'm not sure why it's implemented this way, which is why I've elided this detail in just about every conversation on the topic.) With this in mind, I have a proposal: 1) All parameters of all classes have nominal role. 2) Classes also store one extra bit per parameter, saying whether all uses of that parameter are representational. Essentially, this bit says whether that parameter is suitable for GND. (Currently, we could just store for the last parameter, but we can imagine extensions to the GND mechanism for other parameters.) Yes, this seems right. And NOW I finally realise why GHC implements GND like this. Consider class Show a => C a where { op :: a -> a } instance C Int where ... newtype Age = Age Int deriving( Show, C ) Here we want to make a (C Age) dictionary that use the (C Int) version of 'op'. But the superclass for (Show Age) must not use the (Show Int) version of 'show'! It must use Age's own version of Show. So we I think Richard's proposal is spot on. Go for it. Can you work on that, Richard? Simon From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Richard Eisenberg Sent: 09 October 2013 20:21 To: Edward Kmett Cc: Simon Peyton-Jones; glasgow-haskell-users@haskell.org Mailing List Subject: Re: default roles Now I think we're on the same page, and I *am* a little worried about the sky falling because of this. (That's not a euphemism -- I'm only a little worried.) Well, maybe I should be more worried. The whole idea of roles is to protect against type-unsoundness. They are doing a great job of that here -- no problem that we've discussed in this thread is a threat against type safety. The issue immediately at hand is about coherence (or perhaps you call it confluence) of instances. Roles do not address the issue of coherence at all, and thus they fail to protect against coherence attacks. It would take More Thought to reformulate roles (or devise something else) to handle coherence. It's worth pointing out that this isn't a new problem, exactly. Bug #8338 shows a way to produce incoherence using only the GADTs extension. (It does need 4 modules, though.) I conjecture that incoherence is also possible through GeneralizedNewtypeDeriving, both as it existed in GHC 7.6.3 and in 7.8, so it's not an issue with Coercible, exactly. It's just that Coercible allows you to get incoherence with so much less fuss than before! Wait! I have an idea! The way I've been describing GND all along has been an abbreviation. GHC does not coerce a dictionary from, say, Ord Int to Ord Age. Instead, GHC mints a fresh dictionary for Ord Age where all the methods are implemented as coerced versions of the methods for Ord Int. (I'm not sure why it's implemented this way, which is why I've elided this detail in just about every conversation on the topic.) With this in mind, I have a proposal: 1) All parameters of all classes have nominal role. 2) Classes also store one extra bit per parameter, saying whether all uses of that parameter are representational. Essentially, this bit says whether that parameter is suitable for GND. (Currently, we could just store for the last parameter, but we can imagine extensions to the GND mechanism for other parameters.) Because GND is implemented using coercions on each piece instead of wholesale, the nominal roles on classes won't get in the way of proper use of GND. An experiment (see below for details) also confirms that even superclasses work well with this idea -- the superclasses aren't coerced. Under this proposal, dictionaries can never be coerced, but GND would still seem to work. Thoughts? Richard Experiment: newtype Age = MkAge Int instance Eq Age where _ == _ = False deriving instance Ord Age useOrdInstance :: Ord a => a -> Bool useOrdInstance x = (x == x) What does `useOrdInstance (MkAge 5)` yield? It yields `False` (in HEAD). This means that the existing GND mechanism (I didn't change anything around this part of the code) uses superclass instances for the *newtype*, not for the *base type*. So, even with superclasses, class dictionaries don't need to be coerced. On Oct 9, 2013, at 2:52 PM, Edward Kmett mailto:ekmett@gmail.com> wrote: I'd be happy to be wrong. =) We do seem to have stumbled into a design paradox though. To make it so you can use roles in GeneralizedNewtypeDeriving hinges on the parameter's role being representational, but making it representational means users can also use coerce to turn dictionaries into other dictionaries outside of GND. This is quite insidious, as another dictionary for Eq or Ord may exist for that type, where it becomes unsound as the generated dictionary may be used to destroy confluence. This means that even if something like Set has a nominal argument it isn't safe, because you can attack the invariants of the structure via Ord. newtype Bad = Bad Int deriving Eq instance Ord Bad where compare (Bad a) (Bad b) = compare b a If Ord has a representational role then I can use coerce to convert a dictonary Ord Bad to Ord Int, then work locally in a context where that is the dictionary for Ord Int that I get when I go to do an insert or lookup. I don't mean to sound like the sky is falling, but I do worry that the 'use of a constraint in a data type' may not be necessary or sufficient. That is a lot of surface area to defend against attack. I am not sure that I actually need a data type to coerce a dictionary. It seems likely that I could do it with just a well crafted function argument and ScopedTypeVariables, but my version of HEAD is a bit too mangled at the moment to give it a try. -Edward On Wed, Oct 9, 2013 at 2:09 PM, Richard Eisenberg mailto:eir@cis.upenn.edu> wrote: I don't quite agree with your analysis, Edward. Eq can be auto-derived, so it makes for a confusing example. Let's replace Eq in your example with this class:

class C a where c_meth :: a -> a -> Bool

Then, your example leads to the same embarrassing state of affairs: coercing a dictionary for (C Int) to one for (C Bar). But, I would argue that we still want C's parameter to have a representational role. Why? Consider this:

data Blargh = ... instance C Blargh where ...

newtype Baz = MkBaz Blargh deriving C

We want that last line to work, using GeneralizedNewtypeDeriving. This hinges on C's parameter's role being representational. I think that what you've witnessed is a case of bug #8338 (http://ghc.haskell.org/trac/ghc/ticket/8338). This is a problem, in my view, and it seems to touch on roles, but I'm not completely sure of their relationship. So, I think that classes should keep their representational roles (regardless of the decision on datatypes -- Haskell doesn't really support "abstract" classes), but perhaps we have to find a way to stop these incoherent instances from forming. Maybe the use of a constraint makes a datatype's role be nominal? Richard On Oct 9, 2013, at 1:55 PM, Edward Kmett mailto:ekmett@gmail.com> wrote: I just noticed there is a pretty big issue with the current default role where typeclasses are concerned! When implementing Data.Type.Coercion I had to use the fact that I could apply coerce to the arguments of data Coercion a b where Coercion :: Coercible a b => Coercion a b This makes sense as Coercion itself has two representational arguments. This struck me as quite clever, so I went to test it further. data Foo a where Foo :: Eq a => Foo a newtype Bar = Bar Int instance Eq Bar where _ == _ = False I fully expected the following to fail: coerce (Foo :: Foo Int) :: Foo Bar but instead it succeeded. This means I was able to convert a dictionary Eq Int into a dictionary for Eq Bar! This indicates that Eq (actually all) of the typeclasses are currently marked as having representational, when actually it strikes me that (almost?) none of them should be. Coercible is the only case I can think of in base of a class with two representational arguments, but this is only valid because we prevent users from defining Coercible instances manually. If I try again with a new typeclass that has an explicit nominal role type role Eq nominal class Eq a instance Eq Int instance Eq Bar then I get the failure to derive Coercible (Foo Int) (Foo Bar) that I'd expect. This indicates two big issues to me: 1.) At the very least the default role for type classes should be nominal for each argument. The very point of an instance is to make a nominal distinction after all. =) 2.) It also indicates that making any typeclass with a representational (/ phantom?) argument shouldn't be possible in valid SafeHaskell, as you can use it to subvert the current restrictions on OverlappingInstances. -Edward On Wed, Oct 9, 2013 at 12:07 PM, Iavor Diatchki mailto:iavor.diatchki@gmail.com> wrote: Hello, My preference would be for the following design: 1. The default datatypes for roles are Nominal, but programmers can add annotations to relax this. 2. Generlized newtype deriving works as follows: we can coerce a dictionary for `C R` into `C T`, as long as we can coerce the types of all methods instantiated with `R`, into the corresponding types instantiated with `T`. In other words, we are pretending that we are implementing all methods by using `coerce`. As far as I can see this safe, and matches what I'd expect as a programmer. It also solves the problem with the `Set` example: because `Set` has a nominal parameter, we cannot coerce `Set Int` into `Set MyAge` and, hence, we cannot derive an instance of `MyAge` for `HasSet`. An added benefit of this approach is that when newtype deriving fails, we can give a nicer error saying exactly which method causes the problem. -Iavor On Mon, Oct 7, 2013 at 6:26 AM, Richard Eisenberg mailto:eir@cis.upenn.edu> wrote: As you may have heard, /roles/ will be introduced with GHC 7.8. Roles are a mechanism to allow for safe 0-cost conversions between newtypes and their base types. GeneralizedNewtypeDeriving (GND) already did this for class instances, but in an unsafe way -- the feature has essentially been retrofitted to work with roles. This means that some uses of GND that appear to be unsafe will no longer work. See the wiki page [1] or slides from a recent presentation [2] for more info. [1] : http://ghc.haskell.org/trac/ghc/wiki/Roles [2] : http://www.cis.upenn.edu/~eir/papers/2013/roles/roles-slides.pdf I am writing because it's unclear what the *default* role should be -- that is, should GND be allowed by default? Examples follow, but the critical issue is this: * If we allow GND by default anywhere it is type-safe, datatypes (even those that don't export constructors) will not be abstract by default. Library writers would have to use a role annotation everywhere they wish to declare a datatype they do not want users to be able to inspect. (Roles still keep type-*un*safe GND from happening.) * If we disallow GND by default, then perhaps lots of current uses of GND will break. Library writers will have to explicitly declare when they wish to permit GND involving a datatype. Which do we think is better? Examples: The chief example demonstrating the problem is (a hypothetical implementation of) Set:

module Set (Set) where -- note: no constructors exported!

data Set a = MkSet [a] insert :: Ord a => a -> Set a -> Set a ...

{-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving #-} module Client where

import Set

newtype Age = MkAge Int deriving Eq

instance Ord Age where (MkAge a) `compare` (MkAge b) = b `compare` a -- flip operands, reversing the order

class HasSet a where getSet :: Set a

instance HasSet Int where getSet = insert 2 (insert 5 empty)

deriving instance HasSet Age

good :: Set Int good = getSet

bad :: Set Age bad = getSet

According to the way GND works, `good` and `bad` will have the same runtime representation. But, using Set operations on `bad` would indeed be bad -- because the Ord instance for Age is different than that for Int, Set operations will fail unexpectedly on `bad`. The problem is that Set should really be abstract, but we've been able to break this abstraction with GND. Note that there is no type error in these operations, just wrong behavior. So, if we default to *no* GND, then the "deriving" line above would have an error and this problem wouldn't happen. If we default to *allowing* GND, then the writer of Set would have to include

type role Set nominal in the definition of the Set module to prevent the use of GND. (Why that peculiar annotation? See the linked further reading, above.)

Although it doesn't figure in this example, a library writer who wishes to allow GND in the default-no scenario would need a similar annotation

type role Foo representational to allow it.

There are clearly reasons for and against either decision, but which is better? Let the users decide! Discussion time: 2 weeks. Thanks! Richard _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Does GND make sense in cases where the superclasses aren't also derived? If I had a type T whose Ord instance made use of the Eq instance for some reason, and then I made a newtype T' with a new Eq instance and a GND Ord instance, the calls to (==) in the Ord instance will refer to the T implementation, right? Yes, absolutely. class Show a => C a where op :: a -> a You might want to use GND for the (C Age) instance, but NOT use GND for the Show instance. Simon From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of David Menendez Sent: 10 October 2013 16:25 To: Richard Eisenberg Cc: glasgow-haskell-users@haskell.org Mailing List; Simon Peyton-Jones Subject: Re: default roles On Wed, Oct 9, 2013 at 3:21 PM, Richard Eisenberg mailto:eir@cis.upenn.edu> wrote: Now I think we're on the same page, and I *am* a little worried about the sky falling because of this. (That's not a euphemism -- I'm only a little worried.) Well, maybe I should be more worried. The whole idea of roles is to protect against type-unsoundness. They are doing a great job of that here -- no problem that we've discussed in this thread is a threat against type safety. The issue immediately at hand is about coherence (or perhaps you call it confluence) of instances. Roles do not address the issue of coherence at all, and thus they fail to protect against coherence attacks. It would take More Thought to reformulate roles (or devise something else) to handle coherence. It's worth pointing out that this isn't a new problem, exactly. Bug #8338 shows a way to produce incoherence using only the GADTs extension. (It does need 4 modules, though.) I conjecture that incoherence is also possible through GeneralizedNewtypeDeriving, both as it existed in GHC 7.6.3 and in 7.8, so it's not an issue with Coercible, exactly. It's just that Coercible allows you to get incoherence with so much less fuss than before! Wait! I have an idea! The way I've been describing GND all along has been an abbreviation. GHC does not coerce a dictionary from, say, Ord Int to Ord Age. Instead, GHC mints a fresh dictionary for Ord Age where all the methods are implemented as coerced versions of the methods for Ord Int. (I'm not sure why it's implemented this way, which is why I've elided this detail in just about every conversation on the topic.) With this in mind, I have a proposal: 1) All parameters of all classes have nominal role. 2) Classes also store one extra bit per parameter, saying whether all uses of that parameter are representational. Essentially, this bit says whether that parameter is suitable for GND. (Currently, we could just store for the last parameter, but we can imagine extensions to the GND mechanism for other parameters.) Because GND is implemented using coercions on each piece instead of wholesale, the nominal roles on classes won't get in the way of proper use of GND. An experiment (see below for details) also confirms that even superclasses work well with this idea -- the superclasses aren't coerced. Under this proposal, dictionaries can never be coerced, but GND would still seem to work. Thoughts? Richard Experiment: newtype Age = MkAge Int instance Eq Age where _ == _ = False deriving instance Ord Age useOrdInstance :: Ord a => a -> Bool useOrdInstance x = (x == x) What does `useOrdInstance (MkAge 5)` yield? It yields `False` (in HEAD). This means that the existing GND mechanism (I didn't change anything around this part of the code) uses superclass instances for the *newtype*, not for the *base type*. So, even with superclasses, class dictionaries don't need to be coerced. Does GND make sense in cases where the superclasses aren't also derived? If I had a type T whose Ord instance made use of the Eq instance for some reason, and then I made a newtype T' with a new Eq instance and a GND Ord instance, the calls to (==) in the Ord instance will refer to the T implementation, right? That seems like what'd we'd expect GND to do, but is it ever something you would want to do? -- Dave Menendez mailto:dave@zednenem.com> http://www.eyrie.org/~zednenem/

On Oct 10, 2013, at 1:14 PM, David Menendez wrote:

Sure, but if op uses show internally, we get Int's show, not Age's, right? That seems correct, in that it's doing what GND is supposed to do, but I'll bet it will surprise people.

Yes, you're right. If a method in a subclass uses a superclass method, it uses the base type's instance's method, not the newtype's. Very weird, but I guess it makes sense in its own way. This does show how GND can create instance incoherence even without storing dictionaries in datatypes. Richard

Please see below. On Oct 10, 2013, at 10:09 PM, Edward Kmett wrote:

Wait, that sounds like it induces bad semantics.

Can't we use that as yet another way to attack the sanctity of Set?

class Ord a => Foo a where badInsert :: a -> Set a -> Set a

instance Foo Int where badInsert = insert

newtype Bar = Bar Int deriving (Eq,Foo)

instance Ord Bar where compare (Bar x) (Bar y) = compare y x

Now you can badInsert into a Set.

If that is still in play then even with all the roles machinery then GND doesn't pass the restrictions of "SafeHaskell". =(

Hrm. Yes. I'm out of fresh ideas at the moment. Maybe some will arrive with sleep. Richard

From marlowsd at gmail.com Tue Oct 1 12:39:58 2013 From: marlowsd at gmail.com (Simon Marlow) Date: Tue, 01 Oct 2013 13:39:58 +0100 Subject: Desugaring do-notation to Applicative Message-ID: <524AC29E.9010203@gmail.com> X-List-Received-Date: Sat, 12 Oct 2013 07:42:45 -0000 X-List-Received-Date: Sat, 12 Oct 2013 07:42:45 -0000

Following a couple of discussions at ICFP I've put together a proposal for desugaring do-notation to Applicative: http://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo I plan to implement this following the addition of Applicative as a superclass of Monad, which is due to take place shortly after the 7.8 branch is cut. Please discuss here, and I'll update the wiki page as necessary. Cheers, Simon

From simonpj at microsoft.com Tue Oct 1 13:49:40 2013 From: simonpj at microsoft.com (Simon Peyton-Jones) Date: Tue, 1 Oct 2013 13:49:40 +0000 Subject: Desugaring do-notation to Applicative In-Reply-To: <524AC29E.9010203@gmail.com> References: <524AC29E.9010203@gmail.com> Message-ID: <59543203684B2244980D7E4057D5FBC1485F7075@DB3EX14MBXC306.europe.corp.microsoft.com> X-List-Received-Date: Sat, 12 Oct 2013 07:42:45 -0000 X-List-Received-Date: Sat, 12 Oct 2013 07:42:45 -0000

What happens when there are some monad things that precede the applicative bit: do { x <- e1 ; y <- e2[x] ; z <- e3[x] ; h[y] ... } does this convert to do { x <- e1 ; (y,z) <- (,) <$> e1 <*> e2 ; h[y] ... I assume so, but it would be good to say. Also worth noting that join can be used to eliminate the tuple in arbitrary contexts, not only ones that end in return. Eg in the above example we can desugar to e1 >>= \x -> join (\y z -> do { h[y]; ... }) e2 e3 I think. Again worth documenting if so. I don't know whether the tuple-version or join-version would be more optimisation friendly. Simon | -----Original Message----- | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- | bounces at haskell.org] On Behalf Of Simon Marlow | Sent: 01 October 2013 13:40 | To: glasgow-haskell-users | Subject: Desugaring do-notation to Applicative | | Following a couple of discussions at ICFP I've put together a proposal | for desugaring do-notation to Applicative: | | http://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo | | I plan to implement this following the addition of Applicative as a | superclass of Monad, which is due to take place shortly after the 7.8 | branch is cut. | | Please discuss here, and I'll update the wiki page as necessary. | | Cheers, | Simon | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users at haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

From eir at cis.upenn.edu Tue Oct 1 20:39:16 2013 From: eir at cis.upenn.edu (Richard Eisenberg) Date: Tue, 1 Oct 2013 16:39:16 -0400 Subject: Desugaring do-notation to Applicative In-Reply-To: <59543203684B2244980D7E4057D5FBC1485F7075@DB3EX14MBXC306.europe.corp.microsoft.com> References: <524AC29E.9010203@gmail.com> <59543203684B2244980D7E4057D5FBC1485F7075@DB3EX14MBXC306.europe.corp.microsoft.com> Message-ID: <8A0E5BAE-A047-4F2E-A171-3EE8972051DB@cis.upenn.edu> X-List-Received-Date: Sat, 12 Oct 2013 07:42:46 -0000 X-List-Received-Date: Sat, 12 Oct 2013 07:42:46 -0000

The soundness of this desugaring depends on the Applicatives and Monads following some of the laws. I think that's OK, personally, but this assumption should be made loudly somewhere, and the feature should be opt-in. As far as I know, GHC currently makes no assumptions about lawful class instances, and it might cause very strange failures if this suddenly were to change without opting in. Richard On Oct 1, 2013, at 9:49 AM, Simon Peyton-Jones wrote:

What happens when there are some monad things that precede the applicative bit:

do { x <- e1 ; y <- e2[x] ; z <- e3[x] ; h[y] ... }

does this convert to

do { x <- e1 ; (y,z) <- (,) <$> e1 <*> e2 ; h[y] ...

I assume so, but it would be good to say.

Also worth noting that join can be used to eliminate the tuple in arbitrary contexts, not only ones that end in return. Eg in the above example we can desugar to

e1 >>= \x -> join (\y z -> do { h[y]; ... }) e2 e3

I think. Again worth documenting if so.

I don't know whether the tuple-version or join-version would be more optimisation friendly.

Simon

| -----Original Message----- | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- | bounces at haskell.org] On Behalf Of Simon Marlow | Sent: 01 October 2013 13:40 | To: glasgow-haskell-users | Subject: Desugaring do-notation to Applicative | | Following a couple of discussions at ICFP I've put together a proposal | for desugaring do-notation to Applicative: | | http://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo | | I plan to implement this following the addition of Applicative as a | superclass of Monad, which is due to take place shortly after the 7.8 | branch is cut. | | Please discuss here, and I'll update the wiki page as necessary. | | Cheers, | Simon | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users at haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users at haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

From dag.odenhall at gmail.com Tue Oct 1 20:56:55 2013 From: dag.odenhall at gmail.com (Dag Odenhall) Date: Tue, 1 Oct 2013 22:56:55 +0200 Subject: Desugaring do-notation to Applicative In-Reply-To: <8A0E5BAE-A047-4F2E-A171-3EE8972051DB@cis.upenn.edu> References: <524AC29E.9010203@gmail.com> <59543203684B2244980D7E4057D5FBC1485F7075@DB3EX14MBXC306.europe.corp.microsoft.com> <8A0E5BAE-A047-4F2E-A171-3EE8972051DB@cis.upenn.edu> Message-ID: X-List-Received-Date: Sat, 12 Oct 2013 07:42:46 -0000 X-List-Received-Date: Sat, 12 Oct 2013 07:42:46 -0000

At least Control.Category has RULES that exploit the category laws for optimization. Whether this counts as *GHC* making assumptions, I don't know. :-) On Tue, Oct 1, 2013 at 10:39 PM, Richard Eisenberg <eir at cis.upenn.edu>wrote:

The soundness of this desugaring depends on the Applicatives and Monads following some of the laws. I think that's OK, personally, but this assumption should be made loudly somewhere, and the feature should be opt-in. As far as I know, GHC currently makes no assumptions about lawful class instances, and it might cause very strange failures if this suddenly were to change without opting in.

Richard

On Oct 1, 2013, at 9:49 AM, Simon Peyton-Jones wrote:

...

What happens when there are some monad things that precede the applicative bit:

do { x <- e1 ; y <- e2[x] ; z <- e3[x] ; h[y] ... }

does this convert to

do { x <- e1 ; (y,z) <- (,) <$> e1 <*> e2 ; h[y] ...

I assume so, but it would be good to say.

Also worth noting that join can be used to eliminate the tuple in arbitrary contexts, not only ones that end in return. Eg in the above example we can desugar to

e1 >>= \x -> join (\y z -> do { h[y]; ... }) e2 e3

I think. Again worth documenting if so.

I don't know whether the tuple-version or join-version would be more optimisation friendly.

Simon

| -----Original Message----- | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- | bounces at haskell.org] On Behalf Of Simon Marlow | Sent: 01 October 2013 13:40 | To: glasgow-haskell-users | Subject: Desugaring do-notation to Applicative | | Following a couple of discussions at ICFP I've put together a proposal | for desugaring do-notation to Applicative: | | http://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo | | I plan to implement this following the addition of Applicative as a | superclass of Monad, which is due to take place shortly after the 7.8 | branch is cut. | | Please discuss here, and I'll update the wiki page as necessary. | | Cheers, | Simon | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users at haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users at haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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