Hi Simon On 10 May 2012, at 13:19, Simon Peyton-Jones wrote:

I'm glad you've been trying out kinds. However, I don't understand the feature you want here.

You say:

fromIntgr :: Integer -> BV (size :: D) fromIntgr int = BV mkD int -- doesn't work, but desired.

fromIntgr :: MkD size => Integer -> BV (size :: D) fromIntgr int = BV mkD int -- does work, but is not that useful.

The implementation MUST pass a value parameter for (MkD size =>) to fromIntgr. Your point is presumably that since every inhabitant of kind D is an instance of MkD, the (MkD size =>) doesn't actually constrain the type at all. It really works for every instantiation of 'size'.

So maybe your feature is

Please omit class constraints where I can see that every suitably-kinded argument is an instance of the class.

So we're dealing with the difference between pi and forall. It's clear that "promotion" alone doesn't really deliver the pi behaviour. That still currently requires the singleton construction, which SHE automates, at least in simple cases. (Shameless plug: see my answer on StackOverflow this morning http://stackoverflow.com/questions/10529360/recursively-defined-instances-an...)

I suppose that might be conceivable, but it'd make the language more complicated, and the implementation, and I don't see why the second version is "not that useful".

Start a feature-request ticket if you like, though.

There's a bunch of competing notions to negotiate. Once we have a promotable type, e.g., data Nat = Zero | Succ Nat we get a singleton family data Natty :: Nat -> * where Zeroy :: Natty Zero Succy :: Natty n -> Natty (Suc n) (In fact, SHE has one data family for the singleton construction, and generates suitable data instance declarations, here mapping Nat to Natty.) As Serguey makes clear, we should also get a class like class HasNatty :: Nat -> Constraint where natty :: Natty n instance HasNatty Zero where natty = Zeroy instance HasNatty n => HasNatty (Succ n) where natty = Succy natty Again, SHE automates this construction. The constraint HasNatty n is written (with distinctive ugliness) {:n :: Nat:}, as is the witness, desugaring to the equivalent of (natty :: Natty n). We can then play spot-the-difference between (1) forall (n :: Nat). (2) forall (n :: Nat). Natty n -> (3) forall (n :: Nat). HasNatty n => (1) is genuinely different; (2) and (3) are equivalent but have different pragmatics. (1) does not involve any runtime data, and has stronger free theorems; (2) involved runtime data passed explicitly and readily pattern-matched (for which SHE also has a notational convenience); (3) involves runtime data passed implicitly. (2) is somehow the explicit pi of dependent type theory (and it's how SHE translates pi); (3) is somehow the "implicit pi"; (1) is somehow the "irrelevant pi" (which is an abuse of the letter pi, as the notion is an intersection rather than a product). I'd like to be able to (and SHE lets me) write (2) as pi (n :: Nat). I'd like to be able to (and SHE doesn't let me) write (3) as (n :: Nat) => I see one main dilemma and then finer variations of style. The dilemma is EITHER make forall act like pi for promoted datatypes, so that runtime witnesses are always present OR distinguish pi from forall, and be explicit when runtime witnesses are present Traditional type theory does the former but is beginning to flirt with the latter, for reasons (better parametricity, more erasure) that have value in theory and practice. I regard the latter as a better fit with Haskell in any case. However, it would be good to automate the singleton construction and sugar over the problem, and even better to avoid the singleton construction just using Nat data in place of Natty data. Other dilemmas. If we have have distinct pi and forall, which do we get when we leave out quantifiers? I'd suggest forall, as somehow the better fit with silence and the more usual in Haskell, but it's moot. When we write instance Applicative (Vector n) -- (X) we currently mean, morally, instance forall (n :: Nat). Applicative (Vector n) -- (X) which we can't define, because pure needs the length at runtime. But perhaps we should write instance pi (n :: Nat). Applicative (Vector n) or (if this syntax is unambiguous) instance (n :: Nat) => Applicative (Vector n) both of which would bring n into scope as a runtime witness susceptible to case analysis. There is certainly something to be done (and SHE already does some of it, within the limitations of a preprocessor). I'd be happy to help kick ideas around, if that's useful. All the best Conor

Simon

| -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe- | bounces@haskell.org] On Behalf Of Serguey Zefirov | Sent: 06 May 2012 17:49 | To: haskell | Subject: [Haskell-cafe] Data Kinds and superfluous (in my opinion) | constraints contexts | | I decided to take a look at DataKinds extension, which became available | in GHC 7.4. | | My main concerns is that I cannot close type classes for promoted data | types. Even if I fix type class argument to a promoted type, the use of | encoding function still requires specification of context. I consider | this an omission of potentially very useful feature. | | Example is below. | ------------------------------------------------------------------------ | ----------------- | {-# LANGUAGE TypeOperators, DataKinds, TemplateHaskell, TypeFamilies, | UndecidableInstances #-} {-# LANGUAGE GADTs #-} | | -- a binary numbers. | infixl 5 :* | data D = | D0 | | D1 | | D :* D | deriving Show | | -- encoding for them. | data EncD :: D -> * where | EncD0 :: EncD D0 | EncD1 :: EncD D1 | EncDStar :: EncD (a :: D) -> EncD (b :: D) -> EncD (a :* b) | | -- decode of values. | fromD :: D -> Int | fromD D0 = 0 | fromD D1 = 1 | fromD (d :* d0) = fromD d * 2 + fromD d0 | | -- decode of encoded values. | fromEncD :: EncD d -> Int | fromEncD EncD0 = 0 | fromEncD EncD1 = 1 | fromEncD (EncDStar a b) = fromEncD a * 2 + fromEncD b | | -- constructing encoded values from type. | -- I've closed possible kinds for class parameter (and GHC successfully | compiles it). | -- I fully expect an error if I will try to apply mkD to some type that | is not D. | -- (and, actually, GHC goes great lengths to prevent me from doing that) | -- By extension of argument I expect GHC to stop requiring context with | MkD a where | -- I use mkD "constant function" and it is proven that a :: D. | class MkD (a :: D) where | mkD :: EncD a | instance MkD D0 where | mkD = EncD0 | instance MkD D1 where | mkD = EncD1 | -- But I cannot omit context here... | instance (MkD a, MkD b) => MkD (a :* b) where | mkD = EncDStar mkD mkD | | data BV (size :: D) where | BV :: EncD size -> Integer -> BV size | | bvSize :: BV (size :: D) -> Int | bvSize (BV size _) = fromEncD size | | -- ...and here. | -- This is bad, because this context will arise in other places, some of | which | -- are autogenerated and context for them is incomprehensible to human | -- reader. | -- (they are autogenerated precisely because of that - it is tedious and | error prone | -- to satisfy type checker.) | fromIntgr :: Integer -> BV (size :: D) -- doesn't work, but desired. | -- fromIntgr :: MkD size => Integer -> BV (size :: D) -- does work, but | is not that useful. | fromIntgr int = BV mkD int | ------------------------------------------------------------------------ | ----------------- | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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2012/5/17 Iavor Diatchki :

Hello,

The context in your example serves an important purpose: it records the fact that the behavior of the function may differ depending on which type it is instantiated with.   This is quite different from ordinary polymorphic functions, such as `const` for example,  which work in exactly the same way, no matter how you instantiate them.   Note that it doesn't matter that we can solve the constraint for all types of kind `D`---the constraint is there because we can't solve it _uniformly_ for all types.

Oh, it was of great matter to me. Because constraints like that get through type family expressions and make it hard to conceal state that should satisfy constraints on type family expressions over the type of the state. I can write something like that: data Combinator s a where Combinator :: Class (TypeFamExpr s) => ... -> Combinator s a And I cannot write something like that: data Combinator a where Combinator :: Class (TypeFamExpr s) => .mentions s.. -> Combinator a If my TypeFamExpr does have type variables, I get a wild type error messages that mentions partially computed TypeFamExpr as an argument to constraint. I can make more detailed example, if you wish.

-Iavor PS: As an aside, these two forms of polymorphism are sometimes called "parametric" (when functions work in the same way for all types), and "ad-hoc" (when the behavior depends on which type is being used).

On Sun, May 6, 2012 at 9:48 AM, Serguey Zefirov wrote:

...

I decided to take a look at DataKinds extension, which became available in GHC 7.4.

My main concerns is that I cannot close type classes for promoted data types. Even if I fix type class argument to a promoted type, the use of encoding function still requires specification of context. I consider this an omission of potentially very useful feature.

Example is below.

----------------------------------------------------------------------------------------- {-# LANGUAGE TypeOperators, DataKinds, TemplateHaskell, TypeFamilies, UndecidableInstances #-} {-# LANGUAGE GADTs #-}

-- a binary numbers. infixl 5 :* data D =                D0        |       D1        |       D :* D        deriving Show

-- encoding for them. data EncD :: D -> * where        EncD0 :: EncD D0        EncD1 :: EncD D1        EncDStar :: EncD (a :: D) -> EncD (b :: D) -> EncD (a :* b)

-- decode of values. fromD :: D -> Int fromD D0 = 0 fromD D1 = 1 fromD (d :* d0) = fromD d * 2 + fromD d0

-- decode of encoded values. fromEncD :: EncD d -> Int fromEncD EncD0 = 0 fromEncD EncD1 = 1 fromEncD (EncDStar a b) = fromEncD a * 2 + fromEncD b

-- constructing encoded values from type. -- I've closed possible kinds for class parameter (and GHC successfully compiles it). -- I fully expect an error if I will try to apply mkD to some type that is not D. -- (and, actually, GHC goes great lengths to prevent me from doing that) -- By extension of argument I expect GHC to stop requiring context with MkD a where -- I use mkD "constant function" and it is proven that a :: D. class MkD (a :: D) where        mkD :: EncD a instance MkD D0 where        mkD = EncD0 instance MkD D1 where        mkD = EncD1 -- But I cannot omit context here... instance (MkD a, MkD b) => MkD (a :* b) where        mkD = EncDStar mkD mkD

data BV (size :: D) where        BV :: EncD size -> Integer -> BV size

bvSize :: BV (size :: D) -> Int bvSize (BV size _) = fromEncD size

-- ...and here. -- This is bad, because this context will arise in other places, some of which -- are autogenerated and context for them is incomprehensible to human -- reader. -- (they are autogenerated precisely because of that - it is tedious and error prone -- to satisfy type checker.) fromIntgr :: Integer -> BV (size :: D) -- doesn't work, but desired. -- fromIntgr :: MkD size => Integer -> BV (size :: D) -- does work, but is not that useful. fromIntgr int = BV mkD int

-----------------------------------------------------------------------------------------

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