[GHC] #12452: TemplateHaskell - variables in top level splices and loading modules.

2 Aug 2016


      #12452: TemplateHaskell - variables in top level splices and loading modules.
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           Reporter:  mkloczko       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
  TemplateHaskell                    |
       Architecture:  x86_64         |   Type of failure:  GHC rejects
  (amd64)                            |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following code fails when loading Main module:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module TH where

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax
 import Data.Typeable


 usingType :: a -> Q [Dec]
 usingType _ = do
     let name = "The name!"
     -- theF = "The name!"
     return [FunD (mkName "theF") [Clause [] (NormalB $ LitE $ StringL name
 )  []]]
 }}}

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}

 module Main where
 import TH

 data A = A Int

 -- Runs with `usingType (undefined :: A)` instead
 $(usingType (A 3) )


 main = putStrLn $ theF
 }}}

 The error:

 {{{
 $ ghc --make Main.hs
 [1 of 2] Compiling TH               ( TH.hs, TH.o )
 [2 of 2] Compiling Main             ( Main.hs, Main.o )
 attempting to use module ‘Main’ (Main.hs) which is not loaded
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12452
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