[GHC] #14032: Can't splice TH quote with infix declaration for name in two different namespaces

25 Jul 2017

      #14032: Can't splice TH quote with infix declaration for name in two different
namespaces
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  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:
-------------------------------------+-------------------------------------
 Spun off from #13799 (and
 https://ghc.haskell.org/trac/ghc/ticket/13054#comment:2). This code
 compiles:

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

 infix 5 :*:
 data a :*: b = a :*: b
 }}}

 But this code does not:

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

 $([d| infix 5 :*:
       data a :*: b = a :*: b
     |])
 }}}
 {{{
 $ /opt/ghc/8.2.1/bin/ghc -ddump-splices Bug.hs
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Bug.hs:(4,3)-(6,6): Splicing declarations
     [d| infix 5 :*:_anM, :*:_anL

         data a_anN :*:_anL b_anO = a_anN :*:_anM b_anO |]
   ======>
     infix 5 :*:_a3IL
     infix 5 :*:_a3IK
     data (:*:_a3IK) a_a3IM b_a3IN = a_a3IM :*:_a3IL b_a3IN

 Bug.hs:4:3: error:
     Multiple fixity declarations for ‘:*:_a3IL’
     also at  Bug.hs:(4,3)-(6,6)
   |
 4 | $([d| infix 5 :*:
   |   ^^^^^^^^^^^^^^^...
 }}}

 Inspecting the `-ddump-splices` output reveals why: when `infix 5 :*:` is
 renamed, because `:*:` refers to two different names in both the value and
 type namespaces, it actually gets renamed to `infix 5 :*:_anM, :*:_anL`,
 where `:*:_anM` and `:*:_anL` are the same name with different uniques.

 Normally, this isn't a problem, since feeding `infix 5 :*:_anM, :*:_anL`
 to the typechecker works fine. However, when it's spliced in via Template
 Haskell, it gets fed back into the renamer, where it believes that
 `:*:_anM` and `:*:_anL` are duplicate names, causing the error.

 Really, this is just a symptom of the fact that infix declarations are
 insufficiently powerful to encode information for identical names in
 different namespaces. But until GHC gains this ability, we need to find
 some workaround for this problem. My hunch is that we'll need to give GHC
 the power to recognize these sorts of duplicate fixity declarations in
 `Convert` and only emit one actual fixity declaration per set of
 duplicates. (Disclaimer: I haven't worked out all the fiddly details yet.)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14032
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