[GHC] #15321: Typed holes in Template Haskell splices produce bewildering error messages

28 Jun 2018

      #15321: Typed holes in Template Haskell splices produce bewildering error messages
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Template       |           Version:  8.4.3
  Haskell                            |
           Keywords:  TypedHoles     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If you compile this program with GHC 8.4 or later:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 foo :: String
 foo = test

 bar :: String
 bar = $(_ "baz")
 }}}

 You'll be greeted with a rather strange error message:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:8:7: error:
     • GHC stage restriction:
         ‘foo’ is used in a top-level splice, quasi-quote, or annotation,
         and must be imported, not defined locally
     • In the untyped splice: $(_ "baz")
   |
 8 | bar = $(_ "baz")
   |       ^^^^^^^^^^
 }}}

 `foo` has nothing do with how `bar`'s RHS should be typechecked, so why is
 it being mentioned in the error message?

 In contrast, GHC 8.2 and earlier gives you quite a nice error message:

 {{{
 $ /opt/ghc/8.2.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:8:9: error:
     • Found hole: _ :: [Char] -> Language.Haskell.TH.Lib.ExpQ
     • In the expression: _
       In the expression: _ "baz"
       In the untyped splice: $(_ "baz")
   |
 8 | bar = $(_ "baz")
   |         ^
 }}}

 Tritlo, my hunch is that the valid hole fits stuff is the culprit here. Do
 you think that perhaps when building the subsumption graph, we are trying
 to check the hole's type against that of `foo`, which causes the stage
 restriction error? If so, do you think it is possible to work around this?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15321
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