Visual programming with Elixir: Learning to write binary parsers (2019) | Hacker News Reader

Visual programming with Elixir: Learning to write binary parsers (2019)

hansonkd.medium.com

103 pointsDowwie14 comments

If you are interested in learning about text (not binary) parsing in Elixir check out the nimbleparsec library:

https://hexdocs.pm/nimble_parsec/NimbleParsec.html

I was able to write a very succinct parser for woodworking (using imperial fractions) in around 38 lines of commented code. It parses strings like 0.25 or 2 or 1/4 or 1/4" or 1' 2 3/4" and pops out an integer or float in inches.

I had never used a parser combinator library before but I have written many recursive descent parsers over the years. I was a little surprised when I added the `debug: true` option and saw the shear amount of code it generated. That probably could be reduced by refactoring my combinators.

i am interested in knowing what is woodworking here

Pretty sure he means literal woodworking... You know, taking measurements, hammering, chiseling...

I left out “design” after woodworking. I’m building an online 3D DIY design and instructional app where you can customize plans by entering dimensions in a form (https://plansformer.com/).

some errors there.

  head = 0x00
  piece = <<0x01, 0x01, 0x01, 0x01, 0x01>>
  << head,             # When a variable is already referenced it must be equal for the pattern to match
     piece,            # Match a string
     rest :: binary >> # You can mix matching variables and assigning new variables.

it should have ^head, and ^piece (the term is "pinning"). Unlike in erlang, elixir will happily "reassign" variables to the new content on whatever is on a right side of the match.

I sometimes ponder, how much effort it would take to make a similar matching interface not for binaries but for regexps and/or something grammar-like. After structured matching of binaries regexps in elixir just feel awkward.

It wouldn't work well with other matching constructs, though, if matching is baked into beamvm itself as a core instruction for speed or something. Guess I have something to investigate this evening.

It's not Elixir, but you might be interested in F#'s active patterns, particularly the parameterized active patterns: https://learn.microsoft.com/en-us/dotnet/fsharp/language-ref...

F#'s active patterns allow you to extend the built-in pattern matching to other things, and in this case, you can have patterns that return regular expression matches.

sounds interesting, thanks.

You could probably write a regex parser in pure Elixir

https://kaitai.io/

Worth a look if you are writing binary parsers.

https://archive.is/2rwPv

You can also replace `medium.com` with `scribe.rip` to get a much friendlier frontend: https://hansonkd.scribe.rip/building-beautiful-binary-parser...

Interesting, I didn't know about that one, thanks!

i have a bookmarklet for this. love that it formats the github embeds quite nicely too.

  javascript:(()=>window.location.href="https://scribe.rip"+location.pathname)()

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