[GHC] #13587: addTopDecls fails with typed Template Haskell

18 Apr 2017

      #13587: addTopDecls fails with typed Template Haskell
-------------------------------------+-------------------------------------
           Reporter:  tmcdonell      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.2.1-rc1
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following untyped Template Haskell works as expected:

 {{{#!hs
 --- AddTopDecls.hs ---
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE QuasiQuotes     #-}

 module AddTopDecls where

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax

 importDoubleToDouble :: String -> ExpQ
 importDoubleToDouble fname = do
     n <- newName fname
     d <- forImpD CCall unsafe fname n [t|Double -> Double|]
     addTopDecls [d]
     varE n

 --- Main.hs ---
 {-# LANGUAGE TemplateHaskell #-}

 module Main where

 import AddTopDecls

 main :: IO ()
 main = do
     let sin' = $(importDoubleToDouble "sin")
         cos' = $(importDoubleToDouble "cos")
     --
     print (sin' 0)
     print (cos' pi)
 }}}

 However it fails if I convert to the equivalent typed version:

 {{{#!hs
 --- AddTopDecls.hs ---
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE QuasiQuotes     #-}

 module AddTopDecls where

 import Language.Haskell.TH
 import Language.Haskell.TH.Syntax

 importDoubleToDouble :: String -> Q (TExp (Double -> Double))
 importDoubleToDouble fname = do
     n <- newName fname
     d <- forImpD CCall unsafe fname n [t|Double -> Double|]
     addTopDecls [d]
     unsafeTExpCoerce (varE n)

 --- Main.hs ---
 {-# LANGUAGE TemplateHaskell #-}

 module Main where

 import AddTopDecls

 main :: IO ()
 main = do
     let sin' = $$(importDoubleToDouble "sin")
         cos' = $$(importDoubleToDouble "cos")
     --
     print (sin' 0)
     print (cos' pi)
 }}}

 With the error:

 {{{
...
ghci Main.hs -ddump-splices
 GHCi, version 8.2.0.20170404: http://www.haskell.org/ghc/  :? for help
 [1 of 2] Compiling AddTopDecls      ( AddTopDecls.hs, interpreted )
 [2 of 2] Compiling Main             ( Main.hs, interpreted )
 Main.hs:9:19-44: Splicing expression
     importDoubleToDouble "sin" ======> sin_a4s2
 Main.hs:1:1: Splicing top-level declarations added with addTopDecls
   ======>
     foreign import ccall unsafe "sin" Main.sin :: Double -> Double
Main.hs:9:19: error:
     • GHC internal error: ‘sin_a4s2’ is not in scope during type checking,
 but it passed the renamer
       tcl_env of environment: [a4dl :-> Identifier[sin'::t1, TopLevelLet
 [] False],
                                a4dm :-> Identifier[cos'::t1, TopLevelLet
 [] False],
                                r4cW :-> Identifier[main::IO (),
 TopLevelLet]]
     • In the expression: sin_a4s2
       In the result of the splice:
         $importDoubleToDouble "sin"
       To see what the splice expanded to, use -ddump-splices
       In the Template Haskell splice $$(importDoubleToDouble "sin")
   |
 9 |     let sin' = $$(importDoubleToDouble "sin")
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 Tested with 7.10.3, 8.0.2, and 8.2.0-rc1.

 Unfortunately I can't use untyped TH in my real use case, so if you have
 any suggestions for a workaround that would also be great.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13587
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