On Wed, 26 Mar 2008, Ganesh Sittampalam wrote:
On Wed, 26 Mar 2008, Ross Paterson wrote:
On Wed, Mar 26, 2008 at 08:52:43PM +, Ganesh Sittampalam wrote:
I'm a bit confused about why the following program doesn't compile (in
any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b)
| To use bar, you need (Ord a, Ord b). You're assuming that Ord (a, b)
| implies that, but it's the other way round.
|
| Logically, the implication holds. There's an equivalence:
|
| Ord a /\ Ord b = Ord (a,b)
|
...
| The problem with dictionaries is that you have to store the
You're talking about something else: the dictionaries (Ord a, Ord b) from which the Ord
(a,b) dictionary were constructed. We don't have a very good name for these guys, but
superclass isn't a good one.
Otherwise I agree with all you say. Your idea of using type families is cool.
|
Why unfortunately? Looks fine to me.
Simon
|
| Unfortunately, GHC accepts the following:
|
| {-# LANGUAGE FlexibleInstances #-}
| module Foo2 where
|
| data Foo = Foo
| deriving Eq
|
| instance Ord (Foo, Foo) where
| (Foo, Foo) (Foo, Foo) = False
@haskell.org; Martin Sulzmann
Subject: RE: flexible contexts and context reduction
Why unfortunately? Looks fine to me.
Simon
|
| Unfortunately, GHC accepts the following:
|
| {-# LANGUAGE FlexibleInstances #-}
| module Foo2 where
|
| data Foo = Foo
| deriving Eq
|
| instance Ord (Foo, Foo) where
@haskell.org; Martin Sulzmann
| Subject: RE: flexible contexts and context reduction
|
| Why unfortunately? Looks fine to me.
|
| Simon
|
| |
| | Unfortunately, GHC accepts the following:
| |
| | {-# LANGUAGE FlexibleInstances #-}
| | module Foo2 where
| |
| | data Foo = Foo
| | deriving Eq
@haskell.org; Martin Sulzmann
Subject: RE: flexible contexts and context reduction
Why unfortunately? Looks fine to me.
Simon
|
| Unfortunately, GHC accepts the following:
|
| {-# LANGUAGE FlexibleInstances #-}
| module Foo2 where
|
| data Foo = Foo
| deriving Eq
|
| instance Ord (Foo, Foo) where
If you write the instance
instance Theta = Foo (T a b) where ...
where Theta is an arbitrary context, then, given a dictionary for (Foo (T a
b)), you can get dictionaries for each constraint in Theta. Or in logic
Theta = Foo (T a b)
In your example, Theta is empty.
If
Tom Schrijvers wrote:
On Wed, 26 Mar 2008, Ganesh Sittampalam wrote:
On Wed, 26 Mar 2008, Ross Paterson wrote:
On Wed, Mar 26, 2008 at 08:52:43PM +, Ganesh Sittampalam wrote:
I'm a bit confused about why the following program doesn't compile
(in any of 6.6.1, 6.8.1 and 6.9.20080316).
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
: glasgow-haskell-users@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
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.
but your recursive function requires a recursive constraint,
which your data type does not guarantee by construction,
and which the Ord
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.
but your recursive function requires a recursive constraint, which your data
type does not guarantee byconstruction, and which the
to work on specific Foo values where it is.
-Original Message-
From: Claus Reinke [mailto:[EMAIL PROTECTED]
Sent: 27 March 2008 12:25
To: Sittampalam, Ganesh; Ganesh Sittampalam
Cc: glasgow-haskell-users@haskell.org
Subject: Re: flexible contexts and context reduction
perhaps i'm
On Wed, Mar 26, 2008 at 08:52:43PM +, Ganesh Sittampalam wrote:
I'm a bit confused about why the following program doesn't compile (in
any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b) context
be reduced?
{-# LANGUAGE FlexibleContexts #-}
module Test2 where
foo :: Ord
On Wed, 26 Mar 2008, Ross Paterson wrote:
On Wed, Mar 26, 2008 at 08:52:43PM +, Ganesh Sittampalam wrote:
I'm a bit confused about why the following program doesn't compile (in
any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b) context
be reduced?
To use bar, you need (Ord
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