Infix operators

In Elm, you can define custom infix operators, the purpose of their existence is for providing a more readable version of the code. Ironically, when you're not familiar with the concept, it does the opposite.

(:=) is a custom infix operator, provided by Json.Decode package.

Please consider the following example of a custom infix operator:

import Html exposing (text)

(<|>) : String -> String -> String
(<|>) beginning end =
  beginning ++ "Bar" ++ end

main =
  text ("Foo" <|> "Buz") -- "FooBarBuz"

It is highly recommended to avoid usage of custom infix operators.

Let's get back to (:=) operator.

The type definition for it looks like (:=) : String -> Decoder a -> Decoder a, which means, that we have to pass a String and a Decoder, from the list of available Decoder Primitives and reruns a new decoder, with a string key mapped to it.

Native Code

In JavaScript world, Decoders are callback functions that do type checking.

For example, here is a Decoder String Primitive:

function decodeString(value) {
  if (typeof value === 'string' || value instanceof String) {
    return value;
  }
  crash('a String', value);
}

And here's a JavaScript equivalent of (:=) operator:

function decodeField(field, decoder) {
  return function(value) {
    var subValue = value[field];
    if (subValue !== undefined) {
      return decoder(subValue);
    }
    crash("an object with field '" + field + "'", value);
  };
}

TL;DR

(:=) maps a string key to a callback (it's not exactly a callback, but that's the closest you could think of), which will check the type of the value in JavaScript object, when you convert it into Elm value.