On Wed, 03 Dec 2014 11:59:42 +0000
Simon Marlow
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