nonSequentialActions.js

function numberField (key) {
  return {
    action: "numberField",
    key,
  }
}

function stringField (key) {
  return {
    action: "stringField",
    key,
  }
}

function objectField (key, parser) {
  return {
    action: "objectField",
    key,
    parser,
  }
}

function pure (value) {
  return {
    action: "pure",
    value,
  }
}

// Return a new parser that executes both parsers and apply the result of the
// second one to the function returned by the first one.
function apply (parser1, parser2) {
  return {
    action: "apply",
    parser1,
    parser2,
  }
}

// Map a function to the result of a parser.
function mapParser (parser, f) {
  switch (parser.action) {
    case "pure":
      return pure(f(parser.value))

    default:
      return apply(pure(f), parser)
  }
}

function interpret (parser, json) {
  switch (parser.action) {
    case "pure":
      return parser.value

    case "numberField":
      if (typeof(json[parser.key]) === "number") {
        return json[parser.key]
      } else {
        throw "ParsingError"
      }

    case "stringField":
      if (typeof(json[parser.key]) === "string") {
        return json[parser.key]
      } else {
        throw "ParsingError"
      }

    case "objectField":
      if (typeof(json[parser.key]) === "object") {
        return interpret(parser.parser, json[parser.key])
      } else {
        throw "ParsingError"
      }

    case "apply":
      const f = interpret(parser.parser1, json)
      const x = interpret(parser.parser2, json)
      return f(x)
  }
}

const someJson = {
  database: {
    port: 5432,
    username: "myapp",
  }
}

const parser =
  objectField(
    "database",

    apply(
      apply(
        pure(function (port) {
          return function (username) {
            return `postgresql://${username}:secret@localhost:${port}/myapp`
          }
        }),

        numberField("port")
      ),
      stringField("username")
    )
  )

// To execute the parser, we would call:
// const result = interpret(parser, someJson)

// For later:
export default parser