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

[Li-yao Xia]

Hi Norman,

Try removing the parentheses; they are all redundant but GHC might be a 
bit too strict about them.

Li-yao

On 7/5/19 9:34 AM, Norman Ramsey 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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