elm-order-safe-json-object-decoders

What does this do?

It decodes JSON objects into Elm records without the risk of field order errors.

What's actually even the problem?

Let's say we want to decode this:

userJson : String
userJson =
    """
{
  "firstName": "Ed",
  "lastName": "Kelly",
  "pets": 0
}
"""

Into this:

type alias User =
    { firstName : String
    , lastName : String
    , pets : Int
    }

And we write a decoder using the elm/json and NoRedInk/elm-json-decode-pipeline packages:

import Json.Decode as JD
import Json.Decode.Pipeline as JDP

userDecoder = 
    JD.succeed User
        |> JDP.field "lastName" JD.string
        |> JDP.field "firstName" JD.string
        |> JDP.field "pets" JD.int

Uh oh! Did you spot the mistake?

The implicit constructor User requires us to decode the firstName field first, and the lastName field second - but we've done it the wrong way around.

Because both fields will decode successfully as Strings, there's no way for Elm to detect that we've blundered, so our user will be forever known by the stupid name "Kelly Ed", instead of his rightful and extremely cool name, "Ed Kelly".

So there's no way for Elm to detect this... Ok, move along, nothing to see here.

Hang on, hold my beer.

Right, there actually is a way for Elm to detect this, and that's what this package does.

If you write a similarly broken decoder using this package, you'll get a compiler error like this:

This function cannot handle the argument sent through the (|>) pipe:

118|     JDS.record userConstructor userConstructor
119|         |> JDS.field "lastName" .lastName JD.string
120|         |> JDS.field "firstName" .firstName JD.string
121|         |> JDS.field "pets" .pets JD.int
122|         |> JDS.endRecord
                ^^^^^^^^^^^^^
The argument is:

    JD.Decoder
        { expectedFieldOrder :
              { firstName : Zero
              , lastName : OnePlus Zero
              , pets : OnePlus (OnePlus Zero)
              }
        , gotFieldOrder :
              { firstName : OnePlus Zero
              , lastName : Zero
              , pets : OnePlus (OnePlus Zero)
              }
              -> Bool
        , recordType : { firstName : String, lastName : String, pets : Int }
        , totalFieldCount : OnePlus (OnePlus (OnePlus Zero))
        }

If you know about Peano numbers, this might be clear to you already; if not, I can explain!

Look at the expectedFieldOrder field:

• firstName should be the first field we pass to our constructor function. As we are programmers and we love zero-indexing, let's call it "field zero". So we expect it to have a field order of Zero = 0.
• lastName should be "field one". So it should have a field order of OnePlus Zero = 1.
• pets should be "field two". So it should have a field order of OnePlus (OnePlus Zero) = 2.

Now look at the gotFieldOrder field. Due to mistake we made in writing the decoder:

• firstName actually got a field order of OnePlus Zero = 1.
• lastName actually got a field order of Zero = 0.
• Only pets got the field order we expected, OnePlus (OnePlus Zero) = 2.

So we can see fairly easily(?) that the problem is that we've got the firstName and lastName fields the wrong way around, and we should either edit the code for our decoder or for our constructor to fix this.

Sounds neat! How do I use it?

The API for our decoders is slightly different from the API of the NoRedInk/elm-json-decode-pipeline package.

Polymorphic constructors

The biggest and most important difference is that we don't use the User constructor in our decoder. Instead, we must define a constructor function ourselves as a top-level function.

This is because we need the constructor to be polymorphic. For example, instead of being restricted to take a String for the firstName and lastName fields and an Int for the pets field, each of its fields must be able to take any type of value.

Here's an example of a polymorphic constructor function:

userConstructor : a -> b -> c -> { firstName : a, lastName : b, pets : c }
userConstructor firstName lastName pets =
    { firstName = firstName
    , lastName = lastName
    , pets = pets
    }

API differences

Here are the other API differences from NoRedInk/elm-json-decode-pipeline:

• We open our decoding pipeline with a function called record instead of succeed.

• We pass our polymorphic constructor function into record twice, instead of passing it into succeed once.

• Each time we call our field function, we pass in a record accessor function in addition to the JSON field name and decoder.

• We end our pipeline with a function called endRecord.

And what does that look like in practice?

Here's an example of a decoder:

import Json.Decode as JD
import Json.Decode.Safe as JDS

userDecoder : JD.Decoder User
userDecoder =
    JDS.record userConstructor userConstructor
        |> JDS.field "firstName" .firstName JD.string
        |> JDS.field "lastName" .lastName JD.string
        |> JDS.field "pets" .pets JD.int
        |> JDS.endRecord

Here's an example of a decoder that will fail at compile time, because we've got the firstName and lastName fields in the wrong order.

brokenUserDecoder : JD.Decoder User
brokenUserDecoder =
    JDS.record userConstructor userConstructor
        |> JDS.field "lastName" .lastName JD.string
        |> JDS.field "firstName" .firstName JD.string
        |> JDS.field "pets" .pets JD.int
        |> JDS.endRecord

How does it work?

My friend, that is a story for another day! Suffice to say, it's completely type-safe, it doesn't rely on any evil JavaScript FFI or other weirdness, and it won't crash.