Re: [GHC] #13642: GHCi 8.2 simply ignores TH splice using datatype with a forall'd kind signature

3 May 2017

      #13642: GHCi 8.2 simply ignores TH splice using datatype with a forall'd kind
signature
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: goldfire (added)

Comment:

 After some digging, the function responsible for suppressing the error
 message appears to be `dsWhenNoErrs`:

 {{{#!hs
 dsWhenNoErrs :: DsM a -> (a -> CoreExpr) -> DsM CoreExpr
 dsWhenNoErrs thing_inside mk_expr
   = do { (result, no_errs) <- askNoErrsDs thing_inside
        ; return $ if no_errs
                   then mk_expr result
                   else unitExpr }
 }}}

 For whatever reason, `askNoErrsDs` does not appear to be propagating the
 error message correctly.

 There's also similar issues with splicing in any other features which
 Template Haskell doesn't support. For example, you could just as well use
 `[d| id x = x {-# SCC id #-} |]` in the example above.

 goldfire, do you have an idea of what's going on here?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13642#comment:3
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