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- TURING PROGRAMMING LANGUAGE LIST FIRST N PRIMES FREE
It’s a much cleaner and simpler implementation now, and I’d have to go out of my way to restore the previous behavior.
I didn’t plan for Turing-complete rules, but I’m not going to revert the refactor that introduced it: This allows better error messages and error recovery. In general, it’s a good idea to avoid using them by parsing a more general input and validating it later. Turing-completeness requires the use of dsl::context_* rules, which can be easily audited. They’re syntax sugar for a hand-written parser where you need to specify exactly how everything is parsed, when and how it should backtrack, This is because lexy grammars are not actually declarative: Static constexpr auto rule = dsl :: while_ ( dsl :: any ) dsl::any always succeeds, so dsl::while_() loops forever. With that, executing a while statement is straightforward: Lexy_ext::rewind(rule) parses rule with access to context variables and then rewinds the input back to the original implementation. I just don’t have a reason for it other than supporting a WHILE interpreter.Īs such, this is the only case where I needed to write a custom rule for the example.
This isn’t a technical limitation I could easily change the implementation of dsl::peek() to forward context variables to the inner rules. However, dsl::peek() does not provide access to context variables! Lexy has dsl::peek() for that: it parses a rule but doesn’t consume the input. This means that when we execute the body, we must then rewind the input back to the beginning of the while loop to try again. We branch on var.is_zero() and either skip the body or execute it.īut after executing the body of the loop we might need to do execute it again! Parsing a while statement is similar to if: If_stmt is templated over all possible variable names, skips the common leading if,Īnd then tries to find the matching variable name. I’ve added context variables to lexy for my XML parser example:Īn XML tag has an opening and closing tag, which must be identical: TL DR: I’ve written a lexy grammar that is able to execute, not just parse, a simple Turing-complete language. The parser is thus able to do arbitrary computation while parsing the input. However, in a recent refactoring of the context variables implementation, I’ve accidentally removed a big limitation, which makes lexy Turing-complete: To handle them, I’ve added support for “context variables”: state that can be modified during parsing. Parsers in that category are unable to parse context-sensitive languages such as XML with matching tags. Lexy, being essentially syntax sugar for a recursive descent parser with (manually specified!) arbitrary lookahead but no other state, falls in the latter category.
TURING PROGRAMMING LANGUAGE LIST FIRST N PRIMES FREE
Such parser generators are classified based on the expressiveness of the corresponding formal language.įor example, a strict regular expression can only parse regular languages, which is a strict subset of a deterministic context free language, and so on.
TURING PROGRAMMING LANGUAGE LIST FIRST N PRIMES CODE
You describe how input should be parsed, and lexy generates code for it, taking care of error recovery, parse tree generation, and parse values. I’m currently working on lexy, a C++ parsing DSL library:
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