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Define nullability for alternations.

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This commit is contained in:
Rob Rix 2017-04-07 09:37:13 -04:00
parent c44fb8a5c3
commit 9b69f3aae8
1 changed files with 6 additions and 2 deletions
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8
src/Language/Ruby/Syntax.hs
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@ -89,7 +89,6 @@ type AST grammar = Rose (Node grammar)
-- | Small-step evaluation of an assignment from a grammar onto a syntax.
stepAssignment :: Eq grammar => Assignment grammar a -> [AST grammar] -> Maybe ([AST grammar], a)
stepAssignment = iterFreer (\ assignment yield nodes -> case nodes of
[] -> Nothing -- FIXME: Nullability
Rose Node{..} children : rest -> case assignment of
Rule symbol subRule ->
if symbol == nodeSymbol then
@ -100,7 +99,12 @@ stepAssignment = iterFreer (\ assignment yield nodes -> case nodes of
Children each -> fmap (first (const rest)) (yield each children)
Alt a b -> yield a nodes <|> yield b nodes
-- FIXME: Rule `Alt` Rule `Alt` Rule is inefficient, should build and match against an IntMap instead.
Fail -> Nothing) . fmap ((Just .) . flip (,))
Fail -> Nothing
-- Nullability: some rules, e.g. 'pure a' and 'many a', should match at the end of input.
[] -> case assignment of
Alt a b -> yield a [] <|> yield b []
_ -> Nothing)
. fmap ((Just .) . flip (,))
instance Alternative (Assignment symbol) where