virtual-insurance-products / simple-parser

simple-parser.asd

Simple Parser

This is a very simple common lisp parsing framework. I think that it is (more or less at least) functionally equivalent to things like Haskell’s parsec library. It is a parser combinator library.

A parser constructed in this library is a function called in a parser context (which uses CL dynamic variables bound by #’execute-parser) and which may consume 0 or more characters from the input string and is expected to return a non nil value if it succeeds.

Since, when run in the context of a dynamically bound parser state, a parser is just a function we can use normal CL special forms like or, if and when in building up parsers.

Calling sp-error when something is missing yields a very descriptive error message describing exactly where in the parser the error happened.

(in-package :simple-parser)

(execute-parser (lambda () (sp-starts-with "Hi"))
                "Hi there"
                :allow-trailing-junk t)

(defun greet ()
  (when (or (sp-starts-with "Hi ")
            (sp-starts-with "Hello "))
    ;; Use a regular expression (must be tied to the start) to get the next bit
    (list 'greet
          (or (sp-scan "^[a-z]+" :ignore-case t)
              (sp-error "Missing person's name")))))

(execute-parser #'greet "Hi John")
(execute-parser #'greet "Hi .")
;; Missing person's name at input character 4
;; =======================
;; Hi <<<HERE>>>.

Finally, there is sp-try which allows optional infinite lookahead by grabbing and saving the parser state and, if the parse inside the sp-try block fails, restoring it. Otherwise the parsers don’t do any lookahead.