Hi Matt. I don't know how bad is this, but here's what I came up with.
In order to be able to ask types to make sure something about values (their
equality), you might want to create a type, which contains a value in its
type-parameter, and then ask that types are equal if you want some equality
property in datatype. Here's an example:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE StandaloneDeriving #-}
module Main where
import GHC.TypeLits
newtype TypeValInt (n::Nat) = TypeValInt Int
deriving (Show)
one :: TypeValInt 1
one = TypeValInt 1
two :: TypeValInt 2
two = TypeValInt 2
data MyP a b = MyP (TypeValInt a, TypeValInt b) (TypeValInt b, TypeValInt a)
deriving (Show)
main :: IO ()
main = do
putStrLn "Hello!"
print (MyP (one, two) (two, one))
-- | this will error:
-- print (MyP (one, two) (one, one))
print (MyPGen (one, two) (two, one))
-- | this will error:
-- print (MyPGen (one, two) (one, one))
class TypeVal (g :: a -> *)
instance TypeVal TypeValInt
data MyPGen a b = forall g. (TypeVal g, Show (g a), Show (g b))
=> MyPGen (g a, g b) (g b, g a)
deriving instance Show (MyPGen a b)
On Mon, Jun 22, 2015 at 1:29 PM, Matt Williams wrote: Dear All, I wonder if/ how this is possible? I have a constructor which takes 2 pairs of type t). However, I want to ensure that the pairs are matched: MyP = MyP (t, t) (t, t) But where the first pair contains the same elements as the second, but
reversed in order. Any help much appreciated. BW,
Matt _______________________________________________
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