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Parser Combinators are Easy

Ben Lovy on February 05, 2019

Let's say we've been sent some brand new points. However, the Point Guru is having a burst of 'creativity' today and has devised a crazy transmiss...

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Mihail Malo • Feb 5 '19 • Edited

Nice grammar you got there.
So... regular.
Would be a shame if something...

const ＨａｐｐｅｎｅｄＴｏＩｔ = input => {
  const re = /\d+|\)\]\[\(|\]\[/g
  const out = [[[]]]
  let set = out[0]
  let point = set[0]
  let match
  while ((match = re.exec(input))) {
    const [m] = match
    switch (m) {
      case ")][(":
        out.push((set = []))
      case "][":
        set.push((point = []))
        continue
    }
    point.push(parseInt(m, 10))
  }
  return out
}
console.log(
  JSON.stringify(
    ＨａｐｐｅｎｅｄＴｏＩｔ(".:{([2 3]][[6 2]][[1 2])][([1 4]][[2 1])][([6 9])}:.")
  )
)

Ben Lovy • Feb 5 '19 • Edited

Point Guru wasn't so bonkers after all ;)

Perhaps a more interesting input would have been warranted

Martin Becker • Feb 6 '19 • Edited

At least for paired tokens, I probably would have used a pushdown automata.

Mihail Malo • Feb 6 '19

I only looked at the separators, not the paired wrappers.
Furthermore I didn't rely on the knowledge that the points are 2-dimensional.

You could say I have a stack of stacks of stacks :v

Ben Lovy • Feb 6 '19

Points could just be space separated, the sepBy() combinator would address that restriction

Mihail Malo • Feb 7 '19 • Edited

 2      number
 3      number
 ][     new point
 6      number
 2      number
 ][     new point
 1      number
 2      number
)][(    new set
 ][     new point
 1      number
 4      number
 ][     new point
 2      number
 1      number
)][(    new set
 ][     new point
 6      number
 9      number

Adam Brandizzi • Feb 7 '19

Man, this is a lovely parser library---and a neat explanation. Thanks for this post!

Mihail Malo • Feb 5 '19

FIRST COMMENT