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Decode Expression

In order to process data from JavaScript in Mint, you must convert it from JavaScript:

  • object { name: "Joe" }
  • array [1, 2, 3]
  • number 0
  • string "Joe"
  • null
  • undefined a Mint typed value.

The decode expression:

decode object as String

...will try to convert the untyped value to a typed value. It returns a Result(Object.Error, ...) , which can be used in a try expression .

You can decode JavaScript values into Mint primitive values ( String , Bool , Number , Time ) simple structures ( Maybe(a) , Set(a) , Map(a,b) , Array(a) ) and records which only have decodable values.

If you try to decode a type which is not supported or try to decode something that is not an Object , you will get a nice error message.

An example of decoding a simple record:

record User {
  name : String,
  age : Number

component Main {
  fun render : Html {
    try {
      object =
        Json.parse("{\"name\": \"John\", \"age\": 30 }")
        |> Maybe.toResult("Decode Error")

      user =
        decode object as User

        <{ }>
    } catch Object.Error => error {
        <{ "Could not decode!" }>
    } catch String => error {
        <{ "Invalid JSON!" }>

In this case, user is a Result(Object.Error, User)

Decoding not supported names

There are times when we want to decode an object containing a key whose name is not a valid record key, for example tag_list . In this case the using keyword can be used to specify the mapping between the object and the record.

Here is an example:

record Post {
  tagList: Array(String) using "tag_list"

When decoding an object as a Post it will look for the tag_list field instead of the tagList field in the object, so this JavaScript Object:

  tag_list: ["a", "b"]

will decode into this record:

  tagList = ["a", "b"]
} will use this key for the encoding as well.