[Haskell-cafe] recursive datakinds

29 May 2015

      Hello,

If I were trying to do something like creating types like....
...
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
...
data MyList a where
  MyEnd :: MyList '[]
  MyCons :: a -> MyList b -> MyList (a ': b)
...
mysimplelist :: MyList '[Integer]
mysimplelist = MyCons 1 (MyEnd)
what about
...
myrecursivelist = MyCons 1 myrecursivelist
Cafe2.lhs:23:30:
    Occurs check: cannot construct the infinite type: b ~ a : b
    Expected type: MyList b
      Actual type: MyList (a : b)
    Relevant bindings include
      myrecursivelist :: MyList (a : b) (bound at Cafe2.lhs:23:3)
    In the second argument of 'MyCons', namely 'myrecursivelist'
    In the expression: MyCons 1 myrecursivelist

can I "fix" this somehow?

I obviously want the type of myrecursivelist to be
MyList  '[Integer, Integer, Integer, Integer, Integer, Integer, Integer, Integer,.....]

Something like a completely nonsensical...

MyList  (Fix (Integer ': ))

P.S.

Yes, I barely understand "fix" as a function, let alone a data type.
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[Haskell-cafe] recursive datakinds

Nicholls, Mark