Re: [Haskell-cafe] Splicing type signature in TH

16 Oct 2014

      Hi,

Yes, Michael's hack would do part of the trick, but I forgot to exemplify
part of my question in the example code.

The whole idea is that I would like to be able to splice the generated Type
into a TH quotation. Consider one of the world's most complicated identity
functions:

{-# LANGUAGE TemplateHaskell #-}

import Data.Typeable
import Language.Haskell.Meta.Parse (parseType)
import Language.Haskell.TH

f :: Q Type -> Q Exp
f m = m >>= \t -> SigE (VarE 'id) (FunT t t)

liftT :: Typeable a => a -> Q Type
liftT x = either fail return $ parseType $ show $ typeOf x

-- separate files

splicef :: Typeable a => a -> a
splicef x = $(f (liftT x)) x

This code does not work because liftT depends on the value of x, exposing
TH's state restrictions, but since we only need the type of x to evaluate
liftT, we should be fine.

Thanks,
hugo

On Thu, Oct 16, 2014 at 1:22 AM, Michael Sloan  wrote:
...
Here's a hacky solution to this, using haskell-src-meta to parse the type:
{-# LANGUAGE TemplateHaskell #-}
import Data.Typeable
import Language.Haskell.Meta.Parse (parseType)
import Language.Haskell.TH
liftT :: Typeable a => a -> Type
liftT x = either error id $ parseType $ show $ typeOf x
This doesn't handle qualification properly, as the instance of Show
for TypeRep doesn't qualify names.  A proper solution would involve
directly writing a (TypeRep -> Type) function.
On Wed, Oct 15, 2014 at 9:06 PM, Hugo Pacheco  wrote:
...
Hi list,
I am 99.9% sure that this is currently not possible, but I might as well
ask:
Is there a way to lift type variables into Template Haskell type splices?
What I had in mind would be something like this (erroneous) code:
liftT :: a -> Q Type
liftT (_::a) = [t| $a |]
I have no idea how hard it would be to implement such a feature, or if
it is
remotely doable.
Naively, it seems to me that TH would have to delay evaluating the splice
until the type variable is fully expanded, but all the necessary
information
would still be available at some point during compilation.
Cheers,
hugo
--
www.cs.cornell.edu/~hpacheco
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