Proposal: Overloaded Quotes for Template Haskell

[Brent Yorgey]

On Fri, Dec 30, 2011 at 05:50:18PM -0800, Michael D. Adams wrote:
> On Fri, Dec 30, 2011 at 5:05 PM, Simon Peyton-Jones
> <simonpj at microsoft.com> wrote:
> > | Currently quote forms have type "Q Exp" and nested splices expect
> > | their contents to have type "Q Exp".  I propose that quote forms have
> > | type "forall m. Quasi m => m Exp" and nested splices expect a body of
> > | type "m Exp" where the "m" of the nested splice is the same as the "m"
> > | of the quote form that it is inside of.
> >
> > Happily they already have that type.  Have a look in Language.Haskell.TH.Syntax:
> >   newtype Q a = Q { unQ :: forall m. Quasi m => m a }
> >
> > You want a function from "Q Exp" to "forall m. Quasi m => m Exp".  It would have type
> >
> >   Q Exp -> forall m. Quasi m => m Exp
> >
> > or, isomorphically
> >
> >   forall m. Quasi m => Q Exp -> m Exp
> >
> > And indeed there is such a function: it's called runQ.
> >
> > So I think what you want is already available.
> 
> If there are no splices in the quote then, yes, that is sufficient.
> However, as there is no function of type "forall m. Quasi m => m Exp
> -> Q Exp", the contents of all splices must be of type "Q Exp".  Thus
> in the expression "[| ... $( foo ) ... |]" there is no way for foo to
> modify the state of the memoization table.

Given Simon's description of Q above, there is such a function:
namely, the constructor Q.  Unfortunately it seems that it is not
exported.

-Brent