[Haskell-cafe] Intersection types for Haskell?
José Miguel Vilaça
jmvilaca at di.uminho.pt
Tue Jan 10 13:44:43 EST 2006
Hi
If I understand your problem than the following is a solution:
--------------------------------------------------
{-# OPTIONS -fglasgow-exts #-}
class Foo a b where
g :: a -> b
type A = {- change the following -} Int
type B = {- change the following -} Char
instance Foo A B where
g a = {- change the following -} ' '
type C = {- change the following -} Float
type D = {- change the following -} String
instance Foo C D where
g c = {- change the following -} ""
f :: (Foo a b, Foo c d) => a -> c -> (b, d)
f x y = (g x, g y)
---------------------------------------------------------
Simply create a class an give instances for the wanted types.
The code above is also in a file in attachment.
cheers
José Miguel Vilaça
Departamento de Informática - Universidade do Minho
jmvilaca at di.uminho.pt
-----Mensagem original-----
De: haskell-cafe-bounces at haskell.org
[mailto:haskell-cafe-bounces at haskell.org] Em nome de Brian Hulley
Enviada: terça-feira, 10 de Janeiro de 2006 18:01
Para: Haskell-cafe
Assunto: [Haskell-cafe] Intersection types for Haskell?
Hi -
I'm wondering if there is any possiblility of getting intersection types
into Haskell. For example, at the moment there is no (proper) typing for:
f g x y = (g x, g y)
Ideally, I'd like to be able to write:
f:: (a -> b & c -> d) -> a -> c -> (b,d)
or
f :: (a -> b a) -> c -> d -> (b c, b d)
which is perhaps clearer and prevents bad types such as (Int -> String &
Int -> Char) by construction.
While it may be impossible (?) to infer such a type for f, would it be
possible to make use of such an annotation (esp since Haskell with GHC
extensions already has arbitary rank polymorphism)?
Also, as a second point, could functional dependencies in type classes be
written using a similar syntax eg instead of
class Insert t c a | c a -> t where
insert :: t -> c a -> c a
we could write:
class Insert (h (c a)) c a where
insert :: h (c a) -> c a -> c a
or
class Insert t@(h (c a)) c a where -- re-using as-pattern syntax
insert :: t -> c a -> c a
to avoid having to have a special syntax just for functional dependencies
and/or to be able to write more complicated fundeps more succinctly?
Regards,
Brian Hulley
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