Use permutation parsers when your fields can be in any order

Note

In functional code, we often opt to Keep successful values instead of throwing them away when validating^ᛦ

But what do we do when our fields can be in any order?

Permutation parsers (Text.Parsec.Perm in the parsec library) can parse elements into a structure. The elements could be of any type and in any order that appear only once, like a permutation, with optional elements.

This is added easily with parsec to an existing parser with the permute function:

passportParser :: Parser Passport
passportParser =
  permute $ -- here we permutate the fields so they can be matched in any order
    Passport <$$> byrParser -- <$$> builds the type
      <||> iyrParser -- <||> required field
      <||> P.try eyrParser
      <||> P.try heightParser
      <||> P.try hairColorParser
      <||> P.try eyeColorParser
      <||> passportIdParser
      <|?> (Nothing, Just <$> countryIdParser) -- <|?> optional field

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