AlternativeParser doesn't work in one case, but works if I switch the order

[papler]

thisWayIsBroken = wsMandatory | ( wsOptional & new CharSetTerminal( ':', '=' ) & wsOptional ); butThisWayWorksOK = ( wsOptional & new CharSetTerminal( ':', '=' ) & wsOptional ) | wsMandatory;

This is for a name-value parser, where the name and value can be separated by either just spaces, or a colon or an equal sign. In case of colon or equal sign, spaces in between the name and colon/equalsign and value are optional.

    //All these examples should be valid
    List<string> examples = new List<string>() {

                " ",
                 "   ",
                ":",
                " :",
                ":  ",
                 "  :  ",
                 "=",
                " =",
                "=  ",
                 "  =  "
    };

    var wsMandatory = new RepeatCharTerminal( char.IsWhiteSpace, 1 );
    var wsOptional = new RepeatCharTerminal( char.IsWhiteSpace, 0 );

    var thisWayIsBroken = wsMandatory |  ( wsOptional & new CharSetTerminal( ':', '=' ) & wsOptional );

    var g = new Grammar();
    g.Inner = thisWayIsBroken;
    foreach( var e in examples )
    {
                var match = g.Match( e );
                Console.WriteLine( "{0}   \"{1}\"", ( match.Success ? "ok  " : "FAIL" ), e );
    }

    var butThisWayWorksOK =  ( wsOptional & new CharSetTerminal( ':', '=' ) & wsOptional ) | wsMandatory;

All of the examples should be passing, but I get:

ok     " "
ok     "   "
ok     ":"
FAIL   " :"
ok     ":  "
FAIL   "  :  "
ok     "="
FAIL   " ="
ok     "=  "
FAIL   "  =  "

[cwensley]

Hi @papler,

Thanks for submitting the issue. This is a known behaviour of the alternative parser and a way recursive descent parsers generally work. The first match will be the one it uses, so you have to order the alternatives in a way so that the "longest match" comes first.

So, since wsMandatory matches the ones that start with a space, it won't even bother trying the other alternative.

[papler]

Thanks for the reply. I strongly considered that this is just me not using it right, since it's such a mature library.

So once it matches some mandatory whitespace, it then doesn't try the alternative, but goes ahead with the "=" or ":". However, doesn't it backtrack? This alternative is unsuccessful - try other ones?

[cwensley]

The problem is that it is successful and reached the end of your parser grammar. Eto.Parse is not backtracking in any way currently.

[papler]

Ah ok, I'll continue writing this grammar with that in mind. Thank you!