[GHC] #14289: Pretty-printing of derived multi-parameter classes omits necessary parentheses

[GHC]

#14289: Pretty-printing of derived multi-parameter classes omits necessary
parentheses
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Parser)                           |
           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:
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 Take this file:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS -ddump-splices #-}

 import Language.Haskell.TH

 class C a b

 main :: IO ()
 main = putStrLn $([d| data Foo a = Foo a deriving (C a) |] >>= stringE .
 show)
 }}}

 When this is compiled, `-ddump-splices` does not faithfully print back
 what was written by the user:

 {{{
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )
 Bug.hs:10:19-77: Splicing expression
     [d| data Foo_a4l1 a_a4l3
           = Foo_a4l2 a_a4l3
           deriving C a_a4l3 |]
       >>= stringE . show
   ======>
     "[DataD [] Foo_6989586621679026555 [PlainTV a_6989586621679026557]
 Nothing [NormalC Foo_6989586621679026556 [(Bang NoSourceUnpackedness
 NoSourceStrictness,VarT a_6989586621679026557)]] [DerivClause Nothing
 [AppT (ConT Main.C) (VarT a_6989586621679026557)]]]"
 }}}

 In particular, this pretty-prints `deriving C a_a4l3`, which doesn't even
 parse correctly. It should print `deriving (C a_a4l3)`. Ultimately, there
 is an off-by-one error in the number of parentheses being printed, since
 if you tweak the original example by adding another set of parentheses:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS -ddump-splices #-}

 import Language.Haskell.TH

 class C a b

 main :: IO ()
 main = putStrLn $([d| data Foo a = Foo a deriving ((C a)) |] >>= stringE .
 show)
 }}}

 Then it pretty-prints exactly one set of parentheses (as opposed to two):

 {{{
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )
 Bug.hs:10:19-79: Splicing expression
     [d| data Foo_a1zI a_a1zK
           = Foo_a1zJ a_a1zK
           deriving (C a_a1zK) |]
       >>= stringE . show
   ======>
     "[DataD [] Foo_6989586621679026522 [PlainTV a_6989586621679026524]
 Nothing [NormalC Foo_6989586621679026523 [(Bang NoSourceUnpackedness
 NoSourceStrictness,VarT a_6989586621679026524)]] [DerivClause Nothing
 [AppT (ConT Main.C) (VarT a_6989586621679026524)]]]"
 }}}

 There are two suspicious parts of the GHC codebase that contribute to this
 bug:

 * This parser production:
 http://git.haskell.org/ghc.git/blob/4364f1e7543b6803cfaef321105d253e0bdf08a4:/compiler/parser/Parser.y#l2162

   This recognizes singleton derived classes that are surrounded by a set
 of parentheses, and "strips off" the parentheses.

 * This `Outputable` instance:
 http://git.haskell.org/ghc.git/blob/4364f1e7543b6803cfaef321105d253e0bdf08a4:/compiler/hsSyn/HsDecls.hs#l1102

   This code detects lists of two or more derived classes and surrounds
 them with extra parentheses to make up for the set that was removed during
 parsing. But this fails to detect the case of `deriving (C a)`, since this
 only contains a single class.

   Indeed, trying to distinguish between, say, `deriving T` and `deriving
 (T)` at this level would be quite tricky.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14289>
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