Question

I’d like to write a parser for hashtags. I have been reading the blog
entries on parsing on the opa blog, but they didn’t cover recursive
parsers and constructions of lists a lot.

Hashtags are used by some social networks (Twitter, Diaspora*)
to tag a post. They consist of a hash sign (#) and an alphanumeric
string such as “interesting” or “funny”. One example of a post using
hashtags:

Oh #Opa, you're so #lovely! (Are you a relative of #Haskell?)

Parsing that would result in ["Opa", "lovely", "Haskell"].

I have tried to do it, but it doesn’t quite what I want. (It could
either only parse one hashtag and nothing else, would fail in an endless
loop or fail because there was input it didn’t understand…)
Additionally, here is a Haskell version that implements it.

Answer 1

To begin with a remark: by posing question in Haskell-terms you’re effectively looking for somebody who knows Opa and Haskell hence decreasing chances of finding a person to answer the question ;). Ok, I’m saying it half jokingly as your comments help a lot but still I’d rather see the question phrased in plain English.

I think a solution keeping the structure of the Haskell one would be something like this:

parse_tags =
  hashtag = parser "#" tag=Rule.alphanum_string -> tag
  notag = parser (!"#" .)* -> void
  Rule.parse_list_sep(true, hashtag, notag)

Probably the main ‘trick’ is to use the Rule.parse_list_sep function to parse a list. I suggest you take a look at the implementation of some functions in the Rule module to get inspiration and learn more about parsing in Opa.

Of course I suggest testing this function, for instance with the following code:

_ =
  test(s) =
    res =
      match Parser.try_parse(parse_tags, s) with
      | {none} -> "FAILURE"
      | {some=tags} -> "{tags}"
    println("Parsing '{s}' -> {res}")
  do test("#123 #test #this-is-not-a-single-tag, #lastone")
  do test("#how#about#this?")
  void

which will give the following output:

Parsing '#123 #test #this-is-not-a-single-tag, #lastone' -> [123, test, this, lastone]
Parsing '#how#about#this?' -> FAILURE

I suspect that you will need to fine tune this solution to really conform to what you want but it should give you a good head start (I hope).