Why not this?
data Pair = forall a. Eq a = Pair {x::a, y::a}
equal :: Pair - Bool
equal (Pair x y) = x == y
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Hm, also, with equality constraints you can make the type parametrized, too:
data Pair a' = forall a. (a ~ a', Eq a) = Pair {x::a, y::a}
equal :: Pair a - Bool
equal (Pair x y) = x == y
On 18 July 2013 13:00, Christopher Done chrisd...@gmail.com wrote:
Why not this?
data Pair = forall a. Eq
I'd use GADT syntax for this:
{-# LANGUAGE GADTs #-}
data Pair a where Pair :: Eq a = {x::a, y::a} - Pair a
Sjoerd
On Jul 18, 2013, at 1:05 PM, Christopher Done chrisd...@gmail.com wrote:
Hm, also, with equality constraints you can make the type parametrized, too:
data Pair a' = forall a.
Good point, classic use-case for GADTs.
On 18 July 2013 13:11, Sjoerd Visscher sjo...@w3future.com wrote:
I'd use GADT syntax for this:
{-# LANGUAGE GADTs #-}
data Pair a where Pair :: Eq a = {x::a, y::a} - Pair a
Sjoerd
On Jul 18, 2013, at 1:05 PM, Christopher Done chrisd...@gmail.com
What I always do is to write it like this:
equal pair@Pair{} = foo pair == bar pair
The {} syntax ensures that it doesn't matter how complex the Pair constructor
is.
Sjoerd
On Jul 18, 2013, at 1:52 PM, harry volderm...@hotmail.com wrote:
All of the proposed solutions seem to rely on pattern
Sjoerd Visscher-2 wrote
equal pair@Pair{} = foo pair == bar pair
Interesting solution, I didn't know you could do that. (Do all those who
suggested GADTs - you can add a type context to the constructor of a regular
data type as well, they don't bring you anything here.)
I've also been
I've also been experiencing this a lot in class instances, such as:
class Foo f where
foo :: a - f a
data Bar f a = Foo f = Bar {bar :: f a}
instance Foo (Bar f) where
foo a = Bar (foo a)
Is there any way to avoid repeating the Foo f constraint in the Bar f
instance?
On Jul 18, 2013, at 2:35 PM, harry volderm...@hotmail.com wrote:
Sjoerd Visscher-2 wrote
equal pair@Pair{} = foo pair == bar pair
Interesting solution, I didn't know you could do that. (Do all those who
suggested GADTs - you can add a type context to the constructor of a regular
data type
Sjoerd Visscher-2 wrote
class Foo f where
foo :: a - f a
data Bar f a = Foo f = Bar {bar :: f a}
instance Foo (Bar f) where
foo a = Bar (foo a)
No, you can only omit it where you provide Foo f in another way.
Which brings me back to my original question - is there any way that
On Thu, Jul 18, 2013 at 9:58 AM, harry volderm...@hotmail.com wrote:
Which brings me back to my original question - is there any way that the
type system could be enhanced, so that the compiler understands that Bar
f
= Foo f without being told so explicitly every time?
No. The point is,
Brandon Allbery wrote
No. The point is, it's not simply a type annotation; it's a *value* (a
dictionary) that must be carried along with the rest of the value somehow.
The compiler can't always work out statically which instances need to be
used with the affected value, so it has to be
+1 for the -XDotPostfixApply proposal
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On 2013-07-18 10:46, harry wrote:
Why not let all types carry the dictionary automatically, or at least
every
time that it's used, if that would incur a memory/performance penalty?
GHC
tells me which context to add when it's missing, so it clearly knows.
I'm not sure the claim in your second
This is exactly what GADTs are for.
-Edward
On Thu, Jul 18, 2013 at 6:54 AM, harry volderm...@hotmail.com wrote:
data Eq a = Pair a = Pair {x::a, y::a}
equal :: Pair a - Bool
equal pair = (x pair) == (y pair)
This code will fail to compile, even with the deprecated DatatypeContexts
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