Decoding Objects

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 {
<div><{ "Could not decode!" }></div>
} catch String => error {
<div><{ "Invalid JSON!" }></div>

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

Decoding not supported names

There are times when we want to decode a key from an object into a record whats name is not supported as a record key for example tag_list. In this case we can use the using keyword 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.