Back to Questions
1.0k
views2
comments
Infinite loop when using FParsec
f#

Hopefully someone out there is familiar with Fparsec. If not: [link:www.quanttec.com]

I'm attempting to create a prarser using FParsec that (among other things) will read each line of a text file and stores it into a list.

This seemed pretty straight forward:

let plines = many restOfLine

let strings = run plines "\n\n\nTest1\nTest2"

Unfortunately, I get this runtime exception:

"The combinator 'many' was applied to a parser that succeeds without consuming input and without changing the parser state in any other way. (If no exception had been raised, the combinator likely would have entered an infinite loop.)"

Does anyone know what I'm doing wrong or have a better solution?

My current work around is pretty silly:

let pline =

parse {

let! first = anyChar

if first = '\n' then

return (first.ToString())

else

let! txt = restOfLine

return (first.ToString()+txt)}

let strings = run (many pline) "\n\nTest1\nTest2"

Also, how do I format the code in this post properly?

.

The problem here is that the `restOfLine` parser succeeds without consuming input if it is called at the end of the input (after the Test2 line). So `many restOfLine` would end up in an infinite loop if it didn't throw an exception. (The next version of FParsec will also state an input location in the exception message.)

There are several ways to workaround this issue if you really need to parse the complete input with a parser similar to `plines`. For example, `manyTill restOfLine eof` would do the trick. It's also easy to define a variant of `restOfLine` that doesn't succeed at the end of input (but does on empty lines):

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

open FParsec.Primitives
open FParsec.CharParsers
open FParsec.Error


let pLine: Parser<_,_> =


    fun state ->


       let mutable str = null


       let newState = state.SkipRestOfLine(true, &str)


       if not (LanguagePrimitives.PhysicalEquality state newState) then


           Reply(str, newState)


       else


           Reply(Error, NoErrorMessages, newState)

With this parser definition in place you could simply define `plines` as `many pLine`.

This thread discusses code formatting on hubFS: [link:cs.hubfs.net]

By on 5/16/2010 1:24 AM ()

Thanks!

By on 5/16/2010 9:54 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper