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Tidy up the nullability rules.

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This commit is contained in:
Rob Rix 2017-04-07 09:44:02 -04:00
parent 9b69f3aae8
commit b8e4cb7471
1 changed files with 6 additions and 9 deletions
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13
src/Language/Ruby/Syntax.hs
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@ -88,8 +88,10 @@ 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
Rose Node{..} children : rest -> case assignment of
stepAssignment = iterFreer (\ assignment yield nodes -> case (assignment, nodes) of
-- Nullability: some rules, e.g. 'pure a' and 'many a', should match at the end of input. Either side of an alternation may be nullable, ergo Alt can match at the end of input.
(Alt a b, nodes) -> yield a nodes <|> yield b nodes -- FIXME: Rule `Alt` Rule `Alt` Rule is inefficient, should build and match against an IntMap instead.
(assignment, Rose Node{..} children : rest) -> case assignment of
Rule symbol subRule ->
if symbol == nodeSymbol then
yield subRule nodes
@ -97,12 +99,7 @@ stepAssignment = iterFreer (\ assignment yield nodes -> case nodes of
Nothing
Content -> yield nodeContent rest
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
-- 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
_ -> Nothing)
. fmap ((Just .) . flip (,))