[Haskell-cafe] datatype contexts

2010-07-26 Thread Gregory Crosswhite 
 I agree with prior discussion on this list that adding contexts to 
datatype declarations seems to be more trouble than its worth, since 
these contexts just have to be added again to every function using the 
datatype.  However, I have often wondered:  why do function *have* to 
have these contexts?  What would it affect in the language if we made 
the contexts be implicit, so that if we have


data Datatype a = Context a = Datatype a

then for function declarations

f :: D a - ...

the context Context a is automatically asserted by the compiler?

Cheers,
Greg

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Re: [Haskell-cafe] datatype contexts

2010-07-26 Thread Ryan Ingram 
There are two types of datatype contexts; haskell'98 contexts (which I
think are terrible), and GHC existential contexts (which I like):

class C a where runC :: a - Int
data C a = T1 a = D1 a

All this does is add a context to the D1 *constructor*; that is:
-- D1 :: C a = a - T1 a

But extracting a value of this type does nothing:

foo :: T1 a - Int
foo (D1 a) = runC a -- compile error

However, putting the context on the RHS as you have done works in GHC
and does the right thing; pattern matching on that constructor now
brings the class into scope.  You can think of the datatype has having
another field which is proof that a is a member of C:

{-# LANGUAGE ExistentialQuantification #-}
data T2 a = C a = D2 a
-- D2 :: C a = a - T2 a  -- same as D1

bar :: T2 a - Int
bar (D2 a) = runC a -- works

  -- ryan

On Mon, Jul 26, 2010 at 7:48 AM, Gregory Crosswhite
gcr...@phys.washington.edu wrote:
  I agree with prior discussion on this list that adding contexts to datatype
 declarations seems to be more trouble than its worth, since these contexts
 just have to be added again to every function using the datatype.  However,
 I have often wondered:  why do function *have* to have these contexts?  What
 would it affect in the language if we made the contexts be implicit, so that
 if we have

    data Datatype a = Context a = Datatype a

 then for function declarations

    f :: D a - ...

 the context Context a is automatically asserted by the compiler?

 Cheers,
 Greg

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Re: [Haskell-cafe] datatype contexts

2010-07-26 Thread Gregory Crosswhite 
Oh, now I see!  I knew about (and have used) existential contexts, but
somehow I hadn't made the connection that in a sense they are already
equivalent to our intuition for Haskell 98 contexts done right.  :-) 
Thanks!

Any chance of seeing them in Haskell'11?

Cheers,
Greg

On 07/26/10 10:44, Ryan Ingram wrote:
 There are two types of datatype contexts; haskell'98 contexts (which I
 think are terrible), and GHC existential contexts (which I like):

 class C a where runC :: a - Int
 data C a = T1 a = D1 a

 All this does is add a context to the D1 *constructor*; that is:
 -- D1 :: C a = a - T1 a

 But extracting a value of this type does nothing:

 foo :: T1 a - Int
 foo (D1 a) = runC a -- compile error

 However, putting the context on the RHS as you have done works in GHC
 and does the right thing; pattern matching on that constructor now
 brings the class into scope.  You can think of the datatype has having
 another field which is proof that a is a member of C:

 {-# LANGUAGE ExistentialQuantification #-}
 data T2 a = C a = D2 a
 -- D2 :: C a = a - T2 a  -- same as D1

 bar :: T2 a - Int
 bar (D2 a) = runC a -- works

   -- ryan

 On Mon, Jul 26, 2010 at 7:48 AM, Gregory Crosswhite
 gcr...@phys.washington.edu wrote:
   
  I agree with prior discussion on this list that adding contexts to datatype
 declarations seems to be more trouble than its worth, since these contexts
 just have to be added again to every function using the datatype.  However,
 I have often wondered:  why do function *have* to have these contexts?  What
 would it affect in the language if we made the contexts be implicit, so that
 if we have

data Datatype a = Context a = Datatype a

 then for function declarations

f :: D a - ...

 the context Context a is automatically asserted by the compiler?

 Cheers,
 Greg

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 Haskell-Cafe@haskell.org
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Re: [Haskell-cafe] datatype contexts

2010-07-26 Thread Dominique Devriese 
2010/7/26 Ryan Ingram ryani.s...@gmail.com:
 There are two types of datatype contexts; haskell'98 contexts (which I
 think are terrible), and GHC existential contexts (which I like):

See also GADT-style data type declarations [1] and full GADT's [2],
which both behave like GHC existential contexts mentioned above: pattern
matching on them makes available the context constraint.

Dominique

Footnotes:
[1]  
http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/data-type-extensions.html#gadt-style
[2]  
http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/data-type-extensions.html#gadt
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