json-sml

Thought it would be fun to write a json implementation in Standard ML. It was!

JSON

The interface is based on the ruby MultiJson gem. MultiJson.load converts a string to json, and MultiJson.dump converts json to the string representation.

use "parser.sml";
use "json.sml";

val data = JSON.load "{ \"author\" : \"AJ\" }";
val _ = print ((JSON.dump data) ^ "\n");
(* {"author":"AJ"} *)

Parser

The Parser structure is adapted from a McGill University Comp 302 assignment. Building up parsers through combinations leads to amazingly legible parsers. The code is in parser.sml and json.sml.

One gotcha was that infixes do not get included in the context when you open a structure, so they need to be re-declared.

Stats

At ~ 200 LoC its tiny compared to a self-described tiny C lexer like https://github.com/lloyd/yajl which is ~ 2700 LoC. Yajl has a superset of functionaily though so it may be an unfair comparison.

For a performance comparison I compiled a 3 line program squish.sml that loads, dumps, and prints json passed through a pipe (effectively removing unecessary whitespace). I also wrote the equiavlent Node.js program using the built-in JSON.parse and JSON.stringify methods.

$ ruby perf/benchmark.rb
sml
  0.010000   0.130000   1.390000 (  1.287413)
js
  0.020000   0.130000   8.430000 (  8.362186)

The Standard ML program is ~ 6 times faster. The perf/benchmark.json came out of twitter using the chrome inspector for a real-life example. Ran it several times with similar results, but looking forward to a second opinion cause it seems a bit absurd.

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AJ Ostrow, January 2014