# Angstrom parser optimization

**URL:** <https://discuss.ocaml.org/t/angstrom-parser-optimization/1754>\
**Category:** Learning\
**Tags:** angstrom\
**Created:** [March 21, 2018, 9:29pm UTC](https://discuss.ocaml.org/t/angstrom-parser-optimization/1754 "2018-03-21T21:29:03Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![loxs](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/loxs/32/961_2.png) [@loxs](https://discuss.ocaml.org/u/loxs)\
**Post date:** [March 21, 2018, 9:29pm UTC](https://discuss.ocaml.org/t/angstrom-parser-optimization/1754/1 "2018-03-21T21:29:03Z")

</div>

I am a newbie in parsing and I am still not sure if I get all the terminology and after quite some struggle I only managed to write a suboptimal parser, though luckily, one that works.

Here is a simplified version of my problem  
I need to parse lines of a file which look like this:

```auto
    SomeKey SomeValue ;SomeComment
    SomeKey SomeValue  
    ;SomeComment

```

Some of these are optional and we can also have blank lines, so here is where I am at the moment:

```ocaml
open Angstrom
(* skipping definitions of sub-parsers *)

let data_line =
  choice [
    list [comment];
    list [whitespace; comment;];
    list [whitespace; key; whitespace; value; comment];
    list [whitespace; key; whitespace; value;];
    list [whitespace; key; comment];
    list [whitespace; key;];
    list [whitespace];
  ]

```

This works correctly and satisfies all my sample files and the tests (what I think are all the possible combinations), but I think it’s a lot slower than it should be. In order for the parser to identify a `[whitespace; key]` line (which are the most common ones), it has to go over the whole line at least 3 times.

I tried playing with the `Angstrom.Buffered` interface and that’s probably what I need, but I wasn’t able to figure out how to make it compile. I am also not able to find examples (which I can comprehend) on the internet.

---

<div class="post-metadata">

**Author:** ![seliopou](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/seliopou/32/341_2.png) [@seliopou](https://discuss.ocaml.org/u/seliopou)\
**Post date:** [March 23, 2018, 10:30pm UTC](https://discuss.ocaml.org/t/angstrom-parser-optimization/1754/2 "2018-03-23T22:30:54Z")

</div>

I’m not sure if you care to preserve comments in your parser result, but if you did you could implement your parser like this:

```
let lex p =
  p <* whitespace

let data_line =
  let some x = Some x in
  lift3 (fun x y z -> (x, y, z)
    (lex key) 
    (lex value) 
    (lex (option None (comment >>| some))))

let line =
  whitespace *>
  choice
    [ lex comment >>| fun comment -> `Comment comment
    ; data_line >>| fun (k, v, c) -> `Data(k, v, c) ]

```

To eliminate choice entirely, you could use a bit of lookahead:

```
let line =
  whitespace *>
  peek_char_fail
  >>= function
    | ';' -> lex comment >>| fun comment -> `Comment comment
    | _ -> data_line >>| fun (k, v, c) -> `Data(k, v, c)

```

Hope this helps.