Suggestion: parser.failIf(cond, message)

I wish there are kind of this.

parser.failIf(v => someExpression, "error message")

It is a shorthand of this:

parser.chain(v => condition(v) ? P.fail(message) : P.succeed(v))

Isn’t it reasonable?

[wavebeem]

How about the opposite, .assert?

const listParser = P.seq(P.index, P.string("- "), P.regex(...))
  .map(([index, _1, value]) => ({index, value}))
  // Markdown list should be started from multiple of 2 spaces
  .assert(v => v.index.column % 2 === 0, "indentation error")
  .map(...);

[wavebeem]

Version 1.13.0 released!