[GHC] #13968: TH gives "Illegal binding of built-in syntax" error

12 Jul 2017

      #13968: TH gives "Illegal binding of built-in syntax" error
-------------------------------------+-------------------------------------
           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:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If you try to splice in a definition that borrows an already existing
 `Name` using Template Haskell, GHC gives a confusing error message:

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

 import Language.Haskell.TH

 $(pure [ValD (VarP 'succ) (NormalB (ConE 'True)) []])
 }}}

 {{{
 $ /opt/ghc/8.2.1/bin/runghc Bug.hs

 Bug.hs:6:3: error: Illegal binding of built-in syntax: succ
   |
 6 | $(pure [ValD (VarP 'succ) (NormalB (ConE 'True)) []])
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I'm not exactly sure what should happen here. My hunch was that my
 spliced-in `succ` would shadow the `succ` that's already in-scope. But in
 any case, the error message you get currently is quite awful, so at the
 very least that should be improved.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13968
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