Automatic decoders for primitive types and records

[gdotdesign]

This PR adds automatic decoding to the language for primitive types (Array(a), Maybe(a), String, Number, Bool, Time) and records.

• Parser
• Compiler
• Type Checker
• Formatter
• Update runtime
• Tests

The problem

Converting any JavaScript object to a typed structure is difficult, currently Mint solves this the same way as Elm using decoders which can be very verbose:

record Author {
  bio : String,
  following : Bool,
  image : String,
  username : String
}

module Author {
  fun empty : Author {
    {
      bio = "",
      following = false,
      image = "",
      username = ""
    }
  }

  fun decode (object : Object) : Result(Object.Error, Author) {
    with Object.Decode {
      try {
        bio =
          field("bio", string, object)
          |> Result.withDefault("")

        image =
          field("image", string, object)

        following =
          field("following", boolean, object)

        username =
          field("username", string, object)

        Result.ok(
          {
            bio = bio,
            image = image,
            following = following,
            username = username
          })
      } catch Object.Error => error {
        Result.error(error)
      }
    }
  }
}

The use cases are to decode JSON Http requests or websocket messages.

The solution

The solution is to generate decoders for primitive types and records (which only contain primitive types and records), and adding a language construct to decode them.

The syntax looks like this:

decode expression as Type

where the expression must evaluate to Object and the type must be a primitive type or record. The return value is the same as would be for manual decoding Result(Object.Error, a)

Examples

Decoding string from an object:

decode input as String

Decoding a record:

record Entity {
  name: String,
  email: String,
  id: Number
}
...
decode input as Entity

Decoding a record where values come from different keys:

record Entity {
  name: String from "users_name",
  email: String from "users_email",
  id: Number
}
...
decode input as Entity