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| author | Vladislav Zavialov <vlad.z.4096@gmail.com> | 2019-02-01 20:03:54 +0300 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2019-02-23 21:37:52 -0500 |
| commit | e61f6e35e2fffb1e82e9559852481010fe84d8d3 (patch) | |
| tree | 0846f5666bf0553effbfa72facf3b8557f216492 /compiler/deSugar/DsExpr.hs | |
| parent | 6cce36f83aec33d33545e0ef2135894d22dff5ca (diff) | |
| download | haskell-e61f6e35e2fffb1e82e9559852481010fe84d8d3.tar.gz | |
Expression/command ambiguity resolution
This patch removes 'HsArrApp' and 'HsArrForm' from 'HsExpr' by
introducing a new ambiguity resolution system in the parser.
Problem: there are places in the grammar where we do not know whether we
are parsing an expression or a command:
proc x -> do { (stuff) -< x } -- 'stuff' is an expression
proc x -> do { (stuff) } -- 'stuff' is a command
Until we encounter arrow syntax (-<) we don't know whether to parse
'stuff' as an expression or a command.
The old solution was to parse as HsExpr always, and rejig later:
checkCommand :: LHsExpr GhcPs -> P (LHsCmd GhcPs)
This meant polluting 'HsExpr' with command-related constructors. In
other words, limitations of the parser were affecting the AST, and
all other code (the renamer, the typechecker) had to deal with these
extra constructors by panicking.
We fix this abstraction leak by parsing into an intermediate
representation, 'ExpCmd':
data ExpCmdG b where
ExpG :: ExpCmdG HsExpr
CmdG :: ExpCmdG HsCmd
type ExpCmd = forall b. ExpCmdG b -> PV (Located (b GhcPs))
checkExp :: ExpCmd -> PV (LHsExpr GhcPs)
checkCmd :: ExpCmd -> PV (LHsCmd GhcPs)
checkExp f = f ExpG -- interpret as an expression
checkCmd f = f CmdG -- interpret as a command
See Note [Ambiguous syntactic categories] for details.
Now the intricacies of parsing have no effect on the hsSyn AST when it
comes to the expression/command ambiguity.
Future work: apply the same principles to the expression/pattern
ambiguity.
Diffstat (limited to 'compiler/deSugar/DsExpr.hs')
| -rw-r--r-- | compiler/deSugar/DsExpr.hs | 2 |
1 files changed, 0 insertions, 2 deletions
diff --git a/compiler/deSugar/DsExpr.hs b/compiler/deSugar/DsExpr.hs index d9af622b65..c6ba18b1eb 100644 --- a/compiler/deSugar/DsExpr.hs +++ b/compiler/deSugar/DsExpr.hs @@ -752,8 +752,6 @@ ds_expr _ (HsTickPragma _ _ _ _ expr) = do -- HsSyn constructs that just shouldn't be here: ds_expr _ (HsBracket {}) = panic "dsExpr:HsBracket" -ds_expr _ (HsArrApp {}) = panic "dsExpr:HsArrApp" -ds_expr _ (HsArrForm {}) = panic "dsExpr:HsArrForm" ds_expr _ (EWildPat {}) = panic "dsExpr:EWildPat" ds_expr _ (EAsPat {}) = panic "dsExpr:EAsPat" ds_expr _ (EViewPat {}) = panic "dsExpr:EViewPat" |