Re: [Haskell-cafe] Transforming a ADT to a GADT

14 Sep 2012

      On Fri, Sep 14, 2012 at 2:27 PM, Erik Hesselink  wrote:
...
In general, I think you have to work inside an existential. So you
hide the type of the parsed Term inside an existential. If you want to
apply functions to this Term, you unpack, call the function, and
repack.
Maybe I should expand what I mean by this. Let's say you have:

      data SomeTerm where
        SomeTerm :: Term a -> SomeTerm

Your typecheck function goes:

    typecheck :: Exp -> SomeTerm

and you want to apply:

    transform :: Term t -> Term t

You should do something like:

    f (SomeTerm t) = SomeTerm (transform t)

Or, more generally:

    onSomeTerm :: (forall t. Term t -> Term t) -> SomeTerm -> SomeTerm
    onSomeTerm f (SomeTerm t) =  SomeTerm (f t)

Erik