When I was learning Elm, json decoding stumps me. The documentation says upfront

type Decoder a

A value that knows how to decode JSON values.

And followed by a rather imperative description of Json.Decode.float

float : Decoder Float

Decode a JSON number into an Elm Float.

I suspect my head was parsing as

  • Json.Decode.Decoder a means a decoder that knows how to decode a given json string into a value of type a
  • Json.Decode.Decoder Float knows how to decode a given json string into a Float.
  • If I create my own Json.Decode.Decoder User, it is something that knows how to decode a given json string into a User

I wasn’t aware how deeply OOP had shaped my thinking. Or maybe it’s because of the “-er” suffix of Decoder and my Go instincts. So, I kept wanting to give a string to my Json.Decode.Decoder User, “here, decode it into User – but it can’t

Json.Decode.float.decodeString(someString) -- oh no, not a thing

To ease into the right intuition, it is better I don’t treat Json.Decode.Decoder Float nor Json.Decode.Decoder User as “objects”. They don’t “know how to do” anything. They hold values and are easier to grok if perceived as dumb values like { kind = "Float" } or { kind = "User" }.

A Json.Decode.Decoder a

  • is not an “object”
  • does not have a “method”
  • to perform the work when “given a String”

Give that string to Json.Decode.decodeString function instead:

Json.Decode.decodeString Json.Decode.float someString

The code doing the work is inside the Json.Decode.decodeString function, not with your Json.Decode.Decoder a value.

Our Json.Decode.float decoders are just values (or flags or settings or config… whatever works for your mental model) that will be used to if-else our way inside the Json.Decode.decodeString function, to decide what code branch to run. i.e. a pretend implementation of decodeString might be

decodeString decoder someString =
    case decoder.kind of
        "Float" ->
            String.toFloat someString
        "User" ->

So, any function returning a Decoder a is just a function that returns a flag or settings or configuration… not returning an “object” with “method” that parses a string.

If you’re struggling with Json.Decode.Decoder, hope this mental model helps.

NOTE: it doesn’t matter if the implementation detail is such that there’s actually a function being carried around inside Json.Decode.Decoder a values; the OOP mental model makes this hard to understand.