flexible contexts and context reduction

[Sittampalam, Ganesh]

Because I want to be able to make Foo values where the parameter type isn't in Ord, too. I just want unFoo to work on specific Foo values where it is.

-----Original Message-----
From: Claus Reinke [mailto:claus.reinke at talk21.com] 
Sent: 27 March 2008 12:25
To: Sittampalam, Ganesh; Ganesh Sittampalam
Cc: glasgow-haskell-users at haskell.org
Subject: Re: flexible contexts and context reduction

perhaps i'm missing something but, instead of trying to deduce conditions from conclusions, why can't you just follow ghc's suggestion, and add the constraints to the constructor?

data Foo a where
   Foo1 :: a -> Foo a
   Foo2 :: (Ord a,Ord b) => Foo a -> Foo b -> Foo (a, b)

(possibly with another constraint on Foo1)

claus

> A closer example to what I was actually doing was this:
> 
> {-# LANGUAGE GADTs #-}
> module Foo where
> 
> data Foo a where
>   Foo1 :: a -> Foo a
>   Foo2 :: Foo a -> Foo b -> Foo (a, b)
> 
> unFoo :: Ord a => Foo a -> a
> unFoo (Foo1 a) = a
> unFoo (Foo2 x y) = (unFoo x, unFoo y)
> 
> [in the real code I did actually use the Ord constraint in the base 
> case]
> 
> The error I get is this:
> 
> Foo.hs:10:20:
>    Could not deduce (Ord a2) from the context ()
>      arising from a use of `unFoo' at Foo.hs:10:20-26
>    Possible fix: add (Ord a2) to the context of the constructor `Foo2'
>    In the expression: unFoo x
>    In the expression: (unFoo x, unFoo y)
>    In the definition of `unFoo': unFoo (Foo2 x y) = (unFoo x, unFoo y)
> 
> Foo.hs:10:29:
>    Could not deduce (Ord b1) from the context ()
>      arising from a use of `unFoo' at Foo.hs:10:29-35
>    Possible fix: add (Ord b1) to the context of the constructor `Foo2'
>    In the expression: unFoo y
>    In the expression: (unFoo x, unFoo y)
>    In the definition of `unFoo': unFoo (Foo2 x y) = (unFoo x, unFoo y)
> 
> Which suggests that GHC has also lost track of the fact that Ord (a, 
> b) is true. But it would certainly be necessary to get from Ord (a, b) 
> to (Ord a, Ord b) to get that to work.


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