[Haskell-cafe] recursive datakinds

[Richard Eisenberg]

GHC stubbornly refuses to allow infinite types, although I don't know of a good theoretical reason why. (From a practical standpoint, with infinite types, GHC would be very likely to hang, though that could be avoided with enough engineering, I think.) Your best bet here is, in my view, not to use promoted [] to index your list type, but instead something like

> data List a = Cons a (List a) | Tail [a]

where the Tail constructor marks a (possibly-empty) infinitely-repeating tail of a list. Instead of the infinite (repeat Integer) list, you would just have Tail [Integer], which is much better behaved. I haven't tried to build an indexed list from such a beast, though...

Richard

On May 29, 2015, at 11:47 AM, "Nicholls, Mark" <nicholls.mark at vimn.com> wrote:

> 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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