[Haskell-cafe] Parsec, updateState, and failure
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? Will it be state after the first updateState, or will failure of p1 roll the update back? Is there any bracket- or try-catch-finally-like mechanism for Parsec? Thanks. -- Dimitry Golubovsky Anywhere on the Web ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec, updateState, and failure
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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Re: [Haskell-cafe] Parsec, updateState, and failure
Dimitry Golubovsky wrote: Hi, If a parser which updated user state fails, will such update be reverted? Without actually checking, I would strongly suspect that yes, if a parser fails, all its state modifications are thrown away. (This is usually what you would want...) Is there any bracket- or try-catch-finally-like mechanism for Parsec? I don't think so. If you find yourself wanting to parse data differently depending on stuff you've already seen, you might find it easier to run a 2-pass parser of some kind. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
