[GHC] #11301: Using GHC's parser and rendering the results is unreasonably difficult

[GHC]

#11301: Using GHC's parser and rendering the results is unreasonably difficult
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        Reporter:  bitemyapp         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHC API           |              Version:  7.10.2
      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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Comment (by bitemyapp):

 >Willing to improve it, but it's very unclear what a good interface is
 supposed to be.

 This is good:

 http://hackage.haskell.org/package/haskell-src-exts-1.17.1/docs/Language-
 Haskell-Exts-Parser.html#v:parseExp

 The code you posted is going to send new people running for the hills (or
 Rust. or OCaml). There's no documentation, no way of knowing why one would
 want a frontend plugin or what problem it is solving for us. Googling the
 error message I got earlier was quite fruitless too.

 We don't really have to spend a lot of time discussing the API. Just
 catching up the GHC API's UX to the standard set by haskell-src-exts is
 sufficient for my purposes. Is there a reason a parser of Haskell source
 code needs to emit IO? If not, then there should be types like this
 available:

 {{{#!hs
 parseExp :: String -> ParseResult Exp
 }}}

 even if there are nittier-grittier underlying functions/facilities
 available.

 Further, why doesn't `ParseResult` in the GHC API have a Show instance?
 Trying to figure out something simple like, "parse this expression from a
 string and print the parse tree to stdout", with GHC's API is way-way-way
 harder than it should be and adding "more stuff" that the user has to
 know-about, care-about, etc. isn't good enough.

 It also looks like this frontend plugin thing doesn't work for REPL use.
 This is also not satisfactory.

 I'm assuming there are serious, difficult technical reasons the API
 provided by GHC doesn't include a simple "just parse the damn string"
 function with a Showable ParseResult. Could someone please explain those
 reasons?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11301#comment:3>
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