Stream: helpdesk (published)

Topic: Macro to generate DSL parser code

Viktor G. (Oct 28 2025 at 02:39):

I want to write a bunch of rules that check strings of ;-separated words like "word1;word2;word3;word4", using logical expressions with braces like: "word1&word2|(!prefix3*&!word4)". Writing this out in regex generates very long and buggy expressions. However, code for each expression looks simple, there is just too many of them, so I want to shorten it. Optimally, just write some macro and apply it to a bunch of string rules to generate functions. What is a common way to do it?

Luke Morris (Oct 28 2025 at 02:59):

Have you considered a parser expression grammar like PEG.jl ?

Viktor G. (Oct 28 2025 at 04:00):

I just don't get their examples and how to define functions based on their AST.

Viktor G. (Oct 29 2025 at 02:26):

So that is what I basically need - generating Julia function from PEG parser. But for now I don't understand, what additional rules are needed to parse prefixes and braces:

using PEG

@rule word = r"\w+" |> w-> :(contains(str, $(w)))
@rule formula = word & operation & word |> calculate
@rule operation = "|", "&"
# rule for "prefix*" ?
# rule for any braces ?

function calculate(expression)
    left, op, right = expression
    sym_op = Symbol(op)
    return :($(sym_op)($(left), $(right)))
end

pattern1 = "hello|world"
@generated function compiled_pattern1(str)
    parse_whole(formula, pattern1)
end

compiled_pattern1("hello there")

# What rules should I add for expressions with braces and prefixes?
# "(jul*|rust)&!(cpp)&(hello|hi)"

Luke Morris (Oct 29 2025 at 04:11):

Hmm, there might be some examples in here that could be useful to you.

Last updated: Nov 07 2025 at 04:42 UTC