Re: [GHC] #13608: Expose the type of quasiquotes

31 May 2017

      #13608: Expose the type of quasiquotes
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        Reporter:                    |                Owner:
  facundo.dominguez                  |  facundo.dominguez
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:  QuasiQuotes
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  12778             |  Differential Rev(s):  Phab:D3610
       Wiki Page:                    |
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Comment (by facundo.dominguez):

 Looks like typed splices and quasiquotes will pose some gotchas.

 {{{
 -- Q.hs
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Q where

 import Data.Proxy
 import Language.Haskell.TH.Syntax

 class C a where
   method :: Proxy a -> Q (TExp a)

 instance C Int where
   method _ = [|| 1 :: Int ||]

 instance C Char where
   method _ = [|| 'a' ||]

 q :: forall a. C a => Q (TExp a)
 q = method (Proxy :: Proxy a)
 }}}

 {{{
 -- testQ.hs
 {-# LANGUAGE TemplateHaskell #-}
 import Q
 main :: IO ()
 main = print ($$(q) `asTypeOf` (0 :: Int))
 }}}

 {{{
 $ ghc --make testQ.hs
 [1 of 2] Compiling Q                ( Q.hs, Q.o )
 [2 of 2] Compiling Main             ( testQ.hs, testQ.o )

 testQ.hs:6:18: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘q’
       prevents the constraint ‘(C a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance C Char -- Defined at Q.hs:14:10
         instance C Int -- Defined at Q.hs:11:10
     • In the expression: q
       In the Template Haskell splice $$(q)
       In the first argument of ‘asTypeOf’, namely ‘$$(q)’
   |
 6 | main = print ($$(q) `asTypeOf` (0 :: Int))
   |                  ^
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13608#comment:13
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