[Haskell-cafe] recursive datakinds

Fri May 29 16:19:15 UTC 2015

My actual use case is a bit more complex than just a repeating [Integer, Integer, Integer, Integer, Integer,...]

And in fact the recursion is not even repeated segments of a list...hmmm....or is it?.....not sure....lets say it isnt...for the moment.

At the moment it feels more like a recursive Tree type... a nice unrecursive one would be

mytree :: MyTree 'Branch '[ 'Leaf Integer, 'Branch '[ 'Leaf String, 'Leaf Bool ] ]
mytree = ....

Obviously we can construct recursive trees with recursive functions...that leads to recursive types...and BOOOM

I'm not sure I understand your example though....or if it helps in the above example....its all "a"s...I need a's and b's.

I know OF "fix"....and how we can use it to stratify (?) recursive structures....so...the question is whether we can "Fix" the above somehow.

There is I believe a "Fix" type....which makes my head hurt even more.

From: Richard Eisenberg [mailto:eir at cis.upenn.edu]
Sent: 29 May 2015 5:01 PM
To: Nicholls, Mark
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] recursive datakinds

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<mailto: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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