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lookAhead consume on failure? #149

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lookAhead consume on failure?#149

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@jamesdbrock
jamesdbrock
opened on Feb 11, 2022

purescript-parsing/src/Text/Parsing/Parser/Combinators.purs

Lines 210 to 214 in 223f66b

-- | Parse a phrase, without modifying the consumed state or stream position.
lookAhead :: forall s a m. Monad m => ParserT s m a -> ParserT s m a
lookAhead p = (ParserT <<< ExceptT <<< StateT) \s -> do
Tuple e _ <- runStateT (runExceptT (unwrap p)) s
pure (Tuple e s)

This seems different from the way that Parsec and Megaparsec work? They consume input in case of failure.

https://hackage.haskell.org/package/megaparsec-9.2.0/docs/Text-Megaparsec.html#v:lookAhead

https://hackage.haskell.org/package/parsec-3.1.15.0/docs/Text-Parsec.html#v:lookAhead

See also purescript-contrib/purescript-string-parsers#73

Activity

  1. jamesdbrock commented on Feb 13, 2022

    @jamesdbrock
    jamesdbrock
    on Feb 13, 2022
    MemberAuthor

    https://markkarpov.com/tutorial/megaparsec.html#notfollowedby-and-lookahead

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