[Haskell-cafe] Help wanted getting old GADT code to compile

[Richard Eisenberg]

Hi Norman,

This is a bug in GHC 8.0.1, fixed in later versions. Dropping the parentheses around your forall-type will fix the problem:

>  TLit  :: forall a . (a -> (TExp a)

GHC uses something of a dumb algorithm for detecting the result of a constructor, and that algorithm got stymied by parentheses. Even today, it gets stymied by, e.g., type families, but at least it knows about parentheses.

I hope this helps!
Richard

> On Jul 5, 2019, at 9:34 AM, Norman Ramsey <nr at cs.tufts.edu> wrote:
> 
> Here is a legacy file that does not compile with GHC 8.0.1:
> 
> 
> {-# LANGUAGE RankNTypes, GADTs, KindSignatures #-}
> 
> module Bookgadt
> where
> 
> data TExp :: * -> * where
>  TLit  :: (forall a . (a -> (TExp a)))
>  TLit' ::             (a -> (TExp a))
> 
> 
> 
> The compiler rejects value constructor TLit with this error message:
> 
>    • Data constructor ‘TLit’ returns type ‘forall a. a -> TExp a’
>        instead of an instance of its parent type ‘TExp a’
> 
> I really want to keep the explicit "forall," as I'm trying to make a
> point for readers not familiar with Haskell.   Does anyone know if
> there is some option or other incantation that would enable this code
> to compile?
> 
> 
> Norman
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