Mark T.B. Carroll wrote:
I've been playing with Text.Parsers.Frisby to see how it stacks against other options and, while it's been great so far, I am finding that I can't encode a grammar where what's acceptable depends on what's already been parsed in some nontrivial way. [...] Is this supposed to not be possible in Frisby, or (quite likely) am I missing something that allows me to?
It's intentionally impossible. Frisby uses a dynamic programming approach that crucially depends on the fact that the grammar in question is context-free (actually something related, but the effect is the same). You're trying to parse a context-sensitive language. Sometimes, you can avoid context-sensitivity if there's a way to parse the token in question regardless of whether it's valid. For example, Pascal is a context-sensitive language because you may not use a variable before it has been declared: procedure Foo(x:Integer) begin y := 1; end; This not a correct Pascal program, nevertheless the parse succeeds just fine. The missing declaration for y will be detected when processing the abstract syntax tree further. The key point is that the shape of the abstract syntax tree doesn't depend on whether y is declared or not. Regards, apfelmus