[GHC] #15865: Typed template haskell and implicit parameters lead to incorrect results

6 Nov 2018

      #15865: Typed template haskell and implicit parameters lead to incorrect results
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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:
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 In a similar vein to #15863 but this time with implicit parameters.

 https://gist.github.com/b6919b13abe0954fdad844e16e0edb48

 {{{
 {-# LANGUAGE ImplicitParams #-}
 {-# LANGUAGE TemplateHaskell #-}
 module A where

 import Language.Haskell.TH
 import Data.List (sortBy)

 sort   :: (?cmp :: a -> a -> Ordering) => [a] -> [a]
 sort    = sortBy ?cmp

 me :: Q (TExp ([Int] -> [Int]))
 me = let ?cmp = compare in [|| sort ||]
 }}}

 In module `A` we quote a value which has an implicit argument but in its
 context we bind the implicit so the type of the quote is the monomorphic
 type.

 {{{
 {-# LANGUAGE ImplicitParams #-}
 {-# LANGUAGE TemplateHaskell #-}
 module B where

 import A

 foo :: [Int] -> [Int]
 foo =
   --let ?cmp = compare in
   $$(me)
 }}}

 When we splice in `me`, we get an error about an unbound implicit
 parameter which is totally bogus as we already bound it in `A`. There is
 also dynamic binding if another implicit parameter with the same name is
 in scope but the type of `me` mentions nothing about implicit parameters
 so this shouldn't be allowed.

 {{{
 B.hs:8:10: error:
     • Unbound implicit parameter (?cmp::Int -> Int -> Ordering)
         arising from a use of ‘sort’
     • In the expression: sort
       In the result of the splice:
         $me
       To see what the splice expanded to, use -ddump-splices
       In the Template Haskell splice $$(me)
   |
 8 | foo = $$(me)
   |          ^^
 Failed, one module loaded.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15865>
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