On 31 May 2008, at 7:12 AM, Dimitry Golubovsky wrote:
Hi,
If a parser which updated user state fails, will such update be
reverted?
Suppose we have two parsers combined with <|>
p = p1 <|> p2
p1 has the following:
p1 = try $ do
... -- getting something from the input stream
updateState (\st -> ...) -- updated state based on what gotten
from the input
x <- p3 -- p3 should see updated state and return something
updateState (\st -> ...) -- updated state again (if p3 succeeded)
return x
If p3 fails, p1 fails too (second updateState will not be reached).
But what will p2 (tried next) see in the user state?
The same thing p1 saw. You can see the implementation in http://
darcs.haskell.org/ghc-6.8/libraries/parsec/Text/ParserCombinators/
Parsec/Prim.hs; you want the parsecPlus function. Furthermore, you
might note that the GenParser type is defined as
newtype GenParser tok st a = Parser (State tok st -> Consumed (Reply
tok st a))
runP (Parser p) = p
data Consumed a = Consumed a --input is consumed
| Empty !a --no input is
consumed
data Reply tok st a = Ok !a !(State tok st) ParseError --
parsing succeeded with "a"
| Error ParseError --
parsing failed
data State tok st = State { stateInput :: [tok]
, statePos :: !SourcePos
, stateUser :: !st
}
A parser that fails doesn't deliver a new state for parsecPlus to
consider using going forward.
jcc
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