using type variables in type declarations inside function

Dennis Raddle

12 Apr 2018 12 Apr '18

3:47 p.m.

Let's say I've written a function on three types. myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ... Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how? D

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Li-yao Xia

12 Apr 12 Apr

3:50 p.m.

New subject: using type variables in type declarations inside function

Hi Dennis, Use ScopedTypeVariables. {-# LANGUAGE ScopedTypeVariables #-} myFunc :: forall a b c. a -> b -> c -- explicit binders ... helper :: a -> [b] On 04/12/2018 03:47 PM, Dennis Raddle wrote:

...

Let's say I've written a function on three types.

myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error.

Is this actually possible, and how?

D

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Brandon Allbery

3:51 p.m.

On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote:

...

myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Dennis Raddle

4:24 p.m.

Thanks! By the way, why do I sometimes have to use forall, and sometimes not? I'm also learning Purescript, and I noticed that the examples use 'forall' in every case. Why would it be different with Purescript? D On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery wrote:

...

On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote:

...
myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Brandon Allbery

4:26 p.m.

I don't know Purescript so couldn't say about that. In standard Haskell you don't need to use forall at all; it's used by this extension and by extensions for existential types. On Thu, Apr 12, 2018 at 4:24 PM, Dennis Raddle wrote:

...

Thanks!

By the way, why do I sometimes have to use forall, and sometimes not?

I'm also learning Purescript, and I noticed that the examples use 'forall' in every case. Why would it be different with Purescript? D

On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery wrote:

...
On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote:

...
myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Francesco Ariis

4:42 p.m.

New subject: using type variables in type declarations inside function

On Thu, Apr 12, 2018 at 04:26:50PM -0400, Brandon Allbery wrote:

...

I don't know Purescript so couldn't say about that. In standard Haskell you don't need to use forall at all; it's used by this extension and by extensions for existential types.

RankNTypes too iirc

Ryan Reich

4:41 p.m.

I too am curious about the forall in ScopedTypeVariables. It seems formally unnecessary, so I assume it is designed to avert some kind of inconsistency with standard behavior? Thinking about other extensions, e.g. FlexibleInstances or MultiParamTypeClasses, it seems like they all give meaning to constructs that are forbidden by the standard, while this one actually changes the standard behavior (for the better, imho) and so requires protection by some nonstandard signifier, i.e. forall. On Apr 12, 2018 13:27, "Dennis Raddle" wrote: Thanks! By the way, why do I sometimes have to use forall, and sometimes not? I'm also learning Purescript, and I noticed that the examples use 'forall' in every case. Why would it be different with Purescript? D On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery wrote:

...

On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote:

...
myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Brandon Allbery

4:43 p.m.

It's just compatibility with the standard: there might be existing code that depends on Haskell 98 not extending the scope of a type variable, so you need to be explicit about which type variables' scope to extend. "forall" is already there for other extensions, and is otherwise a no-op in this situation, so it's a safe way to specify the extended scope. On Thu, Apr 12, 2018 at 4:41 PM, Ryan Reich wrote:

...

I too am curious about the forall in ScopedTypeVariables. It seems formally unnecessary, so I assume it is designed to avert some kind of inconsistency with standard behavior? Thinking about other extensions, e.g. FlexibleInstances or MultiParamTypeClasses, it seems like they all give meaning to constructs that are forbidden by the standard, while this one actually changes the standard behavior (for the better, imho) and so requires protection by some nonstandard signifier, i.e. forall.

On Apr 12, 2018 13:27, "Dennis Raddle" wrote:

Thanks!

By the way, why do I sometimes have to use forall, and sometimes not?

I'm also learning Purescript, and I noticed that the examples use 'forall' in every case. Why would it be different with Purescript? D

On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery wrote:

...
On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote:

...
myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

MigMit

4:57 p.m.

It could still be useful in local definitions. Like something :: a -> ... something = let somethinElse :: forall b. b -> a -> ... Az iPademről küldve 2018. ápr. 12. dátummal, 22:43 időpontban Brandon Allbery írta:

...

It's just compatibility with the standard: there might be existing code that depends on Haskell 98 not extending the scope of a type variable, so you need to be explicit about which type variables' scope to extend. "forall" is already there for other extensions, and is otherwise a no-op in this situation, so it's a safe way to specify the extended scope.

...
On Thu, Apr 12, 2018 at 4:41 PM, Ryan Reich wrote: I too am curious about the forall in ScopedTypeVariables. It seems formally unnecessary, so I assume it is designed to avert some kind of inconsistency with standard behavior? Thinking about other extensions, e.g. FlexibleInstances or MultiParamTypeClasses, it seems like they all give meaning to constructs that are forbidden by the standard, while this one actually changes the standard behavior (for the better, imho) and so requires protection by some nonstandard signifier, i.e. forall.

On Apr 12, 2018 13:27, "Dennis Raddle" wrote: Thanks!

By the way, why do I sometimes have to use forall, and sometimes not?

I'm also learning Purescript, and I noticed that the examples use 'forall' in every case. Why would it be different with Purescript? D

...
On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery wrote:

...
On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle wrote: myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ...

Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type variables whose scope is to be extended with an explicit "forall" in the signature.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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Brandon Allbery
Dennis Raddle
Francesco Ariis
Li-yao Xia
MigMit
Ryan Reich

using type variables in type declarations inside function

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