Re: more parser conflicts?

14 Dec 2014

      On Sat, 13 Dec 2014 15:19:34 +0000
Sergei Trofimovich  wrote:
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On Wed, 03 Dec 2014 11:59:42 +0000
Simon Marlow  wrote:
...
...
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In unrelated work, I saw this scroll across when happy'ing the parser:
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shift/reduce conflicts:  60
reduce/reduce conflicts: 16
These numbers seem quite a bit higher than what I last remember (which
is something like 48 and 1, not 60 and 16). Does anyone know why?
4 of reduce/reduce conflicts are result of exact rule copy:
    https://phabricator.haskell.org/D569
...
reduce/reduce conflicts are bad, especially so since they're 
undocumented.  We don't know whether this introduced parser bugs or not. 
  Mike - could you look at this please?  It was your commit that 
introduced the new conflicts.
Agreed.
11 more reduce/reduce (of left 12) came from single scary rule
added in
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commit bc2289e13d9586be087bd8136943dc35a0130c88
   ghc generates more user-friendly error messages
exp10 :: { LHsExpr RdrName }
...
     | 'let' binds {% parseErrorSDoc (combineLocs $1 $2) $ text
                             "parse error in let binding: missing required 'in'"
                        }
The other rules add shift/reduce conflicts as follows:
-- parsing error messages go below here
{- s/r:1 r/r:0 -}
    | '\\' apat apats opt_asig '->' {% parseErrorSDoc (combineLocs $1 $5) $ text
                                                       "parse error in lambda: no expression after '->'"
{- s/r:1 r/r:0 -}
    | '\\'                                       {% parseErrorSDoc (getLoc $1) $ text
                                                       "parse error: naked lambda expression '\'"
                                                    }
{- s/r:1 r/r:0 -}
    | 'let' binds 'in'                           {% parseErrorSDoc (combineLocs $1 $2) $ text
                                                       "parse error in let binding: missing expression after 'in'"
                                                    }
{- s/r:0 r/r:11 -}
     | 'let' binds                                {% parseErrorSDoc (combineLocs $1 $2) $ text
                                                        "parse error in let binding: missing required 'in'"
                                                     }
{- s/r:0 r/r:0 -}
        | 'let'                                      {% parseErrorSDoc (getLoc $1) $ text
                                                       "parse error: naked let binding"
                                                       }
{- s/r:1 r/r:0 -}
        | 'if' exp optSemi 'then' exp optSemi 'else' {% hintIf (combineLocs $1 $5) "else clause empty" }
{- s/r:2 r/r:0 -}
        | 'if' exp optSemi 'then' exp optSemi        {% hintIf (combineLocs $1 $5) "missing required else clause" }
{- s/r:1 r/r:0 -}
        | 'if' exp optSemi 'then'                    {% hintIf (combineLocs $1 $2) "then clause empty" }
{- s/r:2 r/r:0 -}
        | 'if' exp optSemi                           {% hintIf (combineLocs $1 $2) "missing required then and else clauses"
{- s/r:2 r/r:0 -}
        | 'if'                                       {% hintIf (getLoc $1) "naked if statement" }
{- s/r:0 r/r:0 -}
        | 'case' exp 'of'                            {% parseErrorSDoc (combineLocs $1 $2) $ text
                                                        "parse error in case statement: missing list after '->'"
                                                      }
{- s/r:1 r/r:0 -}
        | 'case' exp                                 {% parseErrorSDoc (combineLocs $1 $2) $ text
                                                       "parse error in case statement: missing required 'of'"
                                                     }
{- s/r:1 r/r:0 -}
        | 'case'                                     {% parseErrorSDoc (getLoc $1) $ text
                                                        "parse error: naked case statement"
                                                      }
Shift/reduces look harmless (like MultiWayIf ambiguity)
as they seem to resolve as shift correctly.
Proposed fix as:
    https://phabricator.haskell.org/D571

Simon, is using happy's "error" token the proper way of fixing
these error reporting rules?

Thanks!

-- 

  Sergei