On Wed, 03 Dec 2014 11:59:42 +0000 Simon Marlow <marlo...@gmail.com> wrote:
> >> In unrelated work, I saw this scroll across when happy'ing the parser:
> >>
> >>> 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
> 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.
--
Sergei
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