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Typed template haskell and implicit parameters lead to incorrect results

    • Open Issue created by Matthew Pickering @mpickering

    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.
    Trac metadata
    Trac field Value
    Version 8.6.1
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
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