Re: [Haskell-cafe] Function signatures and type class constraints

It uses less space. Conceptually:

data X a = Num a => ConX a

looks like this in memory: { ConX tag, a, proof of Num a } where the proof is usually a pointer to the dictionary for that typeclass at that type. Whereas

data Num a => Y a = ConY a

looks like this: { ConY tag, a } This is why the rhs constraint lets you access methods of the typeclass inside the function; pattern matching on ConX provides a Num dictionary for the code to call. For ConY, it has to get that dictionary from somewhere-- the caller has to pass it in. -- ryan On Wed, Aug 25, 2010 at 9:59 AM, Oscar Finnsson wrote:

Thanks for the tip. You saved my day (and code)!

So what is the point of having the constraint on the left side of the '='? Will it allow me to do anything that the right-side constraint won't?

-- Oscar

On Mon, Aug 23, 2010 at 11:06 PM, Daniel Fischer wrote:

...

On Monday 23 August 2010 22:30:03, Oscar Finnsson wrote:

...

Hi,

I'm wondering why I have to repeat the class constraints at every function.

If I got the data type

...

data (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c = Foo a b c

Type class constraints on datatypes are considered a wart. They don't do what people expect, in particular they don't make the constraints available at the use site.

It works if you move the constraints across the '=':

{-# LANGUAGE ExistentialQuantification #-}

data Foo a b c = (Eq a, Show a, ...) => Foo a b c

or with GADT syntax:

{-# LANGUAGE GADTs #-}

data Foo x y z where  Foo :: (Eq a, Show a, ...) => a -> b -> c -> Foo a b c

Both make the constraints available at the use site,

bar :: Foo a b c -> String bar (Foo a b c)    = "Foo " ++ show a ++ " " ++ show b ++ " " ++ show c ++ ", Yay!"

...

and then a function from Foo to String I have to supply the signature

...

bar :: (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c -> String

even though it should be clear that a, b and c *must* fulfill the constraints already so I should be able to just supply the signature

One would think so. It's a wart.

...

...

bar :: Foo a b c -> String

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