[Haskell-cafe] recursive datakinds

Nicholls, Mark nicholls.mark at vimn.com
Fri May 29 15:47:45 UTC 2015


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

    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 ': ))


Yes, I barely understand "fix" as a function, let alone a data type.

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