Indentation tokens in moo.

index.js

const moo = require('moo')

const lexer = moo.compile({
  ws: /[ \t]+/,
  nl: { match: /(?:\r\n?|\n)+/, lineBreaks: true },
  id: /\w+/,
})

// example

const tokens = indented(lexer, `
if this
  if that
    another
else
  there
`)

for (const tok of tokens) console.log(tok)

// implementation

function* indented(lexer, source) {
  let iter = peekable(lexer.reset(source))
  let stack = []

  // absorb initial blank lines and indentation
  let indent = iter.nextIndent()

  for (let tok; tok = iter.next(); ) {
    if (tok.type === 'nl') {
      const newIndent = iter.nextIndent()
      if (newIndent == null) break // eof

      if (newIndent === indent) {
        yield {type: 'nl'}

      } else if (newIndent > indent) {
        stack.push(indent)
        indent = newIndent
        yield {type: 'indent'}

      } else {
        while (newIndent < indent) {
          indent = stack.pop()
          yield {type: 'dedent'}
        }
        if (newIndent !== indent) {
          throw new Error('inconsistent indentation')
        }
      }
      indent = newIndent

    // ignore whitespace within lines
    } else if (tok.type !== 'ws') {
      yield tok
    }
  }

  // dedent remaining blocks at eof
  for (let i = stack.length; i--;) {
    yield {type: 'dedent'}
  }
}

function peekable(lexer) {
  let here = lexer.next()
  return {
    next() {
      const old = here
      here = lexer.next()
      return old
    },
    peek() {
      return here
    },
    nextIndent() {
      for (let tok; tok = this.peek(); ) {
        if (tok.type === 'nl') {
          this.next()
          continue
        }
        if (tok.type === 'ws') {
          const indent = tok.value.length
          this.next()

          const next = this.peek()
          if (!next) return
          if (next.type === 'nl') {
            this.next()
            continue
          }
          return indent
        }
        return 0
      }
    },
  }
}

LICENSE

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>

package.json

{
  "author": "Nathan",
  "license": "Unlicense",
  "dependencies": {
    "moo": "^0.3.3"
  }
}

I’m curious, why did you choose to consume newline tokens when they’re before an indent/dedent?

Notably, Python doesn’t do this.