[GHC] #7396: Improve "parse error on input `$'" when TemplateHaskell PRAGMA not declared

[GHC]

#7396: Improve "parse error on input `$'" when TemplateHaskell PRAGMA not declared
---------------------------------+------------------------------------------
    Reporter:  ydewit            |       Owner:                     
        Type:  feature request   |      Status:  new                
    Priority:  normal            |   Milestone:                     
   Component:  Template Haskell  |     Version:  7.4.2              
    Keywords:                    |          Os:  MacOS X            
Architecture:  x86               |     Failure:  Building GHC failed
  Difficulty:  Unknown           |    Testcase:                     
   Blockedby:                    |    Blocking:                     
     Related:                    |  
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Changes (by simonpj):

  * difficulty:  => Unknown
  * type:  bug => feature request


Old description:

> Spent close to 1h chasing what possibly could go wrong here:
> {{{
> import           Data.Binary
> import           Data.DeriveTH
>
> data Test = Pass | Failed
>
> $(derive makeBinary ''Test)
> }}
>
> Until, stupid me, I realized that the TemplateHaskell PRAGMA was missing.
> Could this message be a bit more friendly pointing me to the real issue?
>
> If fixing this is simple, you could give me a few pointers and I'll try
> to provide a patch even though my Haskell experience is a bit thin.

New description:

 Spent close to 1h chasing what possibly could go wrong here:
 {{{
 import           Data.Binary
 import           Data.DeriveTH

 data Test = Pass | Failed

 $(derive makeBinary ''Test)
 }}}

 Until, stupid me, I realized that the TemplateHaskell PRAGMA was missing.
 Could this message be a bit more friendly pointing me to the real issue?

 If fixing this is simple, you could give me a few pointers and I'll try to
 provide a patch even though my Haskell experience is a bit thin.

--

Comment:

 The error I get is
 {{{
 T7396.hs:5:1: parse error on input `$'
 }}}
 which of course is right, absent TH.  Is that what happened to you?

 I think one could improve the error mesage by parsing a top-level
 declaration splice (which is otherwise not legal Haskell 98), and then
 reporting "Perhaps you meant -XTemplateHaskell" in the renamer, or
 something like that.

 But for expression splices it's not so easy:
 {{{
 f x = g $(foo)
 }}}
 really does mean something in Haskell 98, namely
 {{{
 f x = f $ foo
 }}}
 So in expressions we can't parse as TH and reject later.

 If you want to have a shot at this, by all means.  One difficulty is that
 the ''lexer'' recognises the token "`$(`" as a single lexeme when TH is
 on.  It can't do that when TH is off.  Maybe in the parser you can have a
 new production
 {{{
 topdecl ::= '$' '(' exp ')'
 }}}
 but I have not thought through all the consequences.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7396#comment:1>
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