Arc Forumnew | comments | leaders | submitlogin
4 points by raymyers 6120 days ago | link | parent

Macros that throw syntax errors at macro-expansion -- not a bad idea. I had actually experimented a bit with this.

  arc> (load "lib/treeparse.arc")
  nil
  arc> (let (s clause forms) nil
         (= forms (filt [list:cons 'do _] (many anything)))
         (= clause (filt car (list (seq anything forms))))
         (= s (filt [cons 'if _] (many clause)))
         (mac cond clauses (parse-all s clauses)))
  #3(tagged mac #<procedure>)

  arc> (cond nil 1 t 2)
  Parse error: extra tokens '(nil 1 t 2)
  nil

  arc> (cond (nil 1) (t 2))
  2
Not the most meaningful error message in the word, but it's a start.


1 point by stefano 6119 days ago | link

Interesting. This could eventually evolve.

-----

1 point by raymyers 6119 days ago | link

As I have remarked before, treeparse was influenced by Haskell's parsec. One of the features of parsec is the optional infusion of more meaningful error messages into grammars. One could easily imagine adding this functionality to treeparse.

-----

1 point by almkglor 6119 days ago | link

Interesting. I suppose this means that (at least for the cps version of treeparse) a fail must also "return" an error value (potentially just a "Expected lit %c, not found"). Of course 'alt parsers would ignore the error message, and generate its own when all alternatives fail ("No alternatives found"). Hmm. I think if the fail continuation returned an error message, we could create a filt-style parser (for the cps case):

  (def onerr (parser msg)
    (fn (remaining pass fail)
      (parser remaining pass
        (fn (_) ; ignored message
          (fail msg)))))
For the monadic version, our old nil-as-fail code would have to be changed I suspect; basically instead of the old type Return = Return parsed remaining actions | nil, we'll need type Return = Return parsed remainig actions | Failed message .

-----