Parsec - how to implement recursive grammar?

[AlesSturala]

Hello,

I hope that someone can help me. I'm trying to implement a simple (but recursive) grammar:

E = ( E ) | term

But I cannot figure out how to recursively call itself: E = ( E )

I tried the following but it fails on NullPointerException:

` Parser ex = null;

        ex = from ws in choice(between(lb, rb, ex), number)
                 select ws;

        var res = parse(ex, "( 10 )");
        WriteLine(res.ToString());`

Any help is appreciated, thank you in advance.

[louthy]

So, here's a few tips:

1. Declare ex before-hand as null

   Parser<YourExpr> ex = null;

2. The first line of the LINQ expression can't refer to ex

    ex = from ... in THIS_WILL_BE_EVALUATED_IMMEDIATELY
         from ... in THIS_WILL_BE_EVALUATED_LAZILY
         select ...;

Because the right-hand-side of the first in is evaluated immediately, ex will be null. And so, your between parser won't be particularly useful.

You can fix this (in general), by using lazyp(() => ex), which will evaluate ex later, when it's got a value.

    ex = from ws in choice(between(lb, rb, lazyp(() => ex)), number)
         select ws;

Or, move the recursion to the second line of the parser, this can be done by consuming the unitp parser:

    ex = from _ in unitp
         from ws in choice(between(lb, rb, ex), number)
         select ws;

3. Take a look at some examples:

The AccountingDSL sample has a script-parser, which can parse scripts like this:

TotalAssets = sum(filter("Asset", rows));
TotalExpenses = sum(filter("Expense", rows));
Balance = TotalAssets - TotalExpenses;
log("Total Assets : {TotalAssets}");
log("Total Expenses : {TotalExpenses}");
log("Balance (Total Assets - Total Expenses): {Balance}");