You could have designed the Json.Decode library!

A lot of people who are learning elm for the first time find the elm's Json.Decode library to be a big stumbling block. It's the only way to write JSON values from elm, so you're probably going to have to use it if you want your elm program to talk to the outside world.

But why does it have to be so, well, weird?

You've got the hang of how to define records and tagged unions already, and you had to learn how to use case and pattern matching to read complicated data structures already -- what's up with Json.Decode making you build up decoders using a special set of functions like field and oneOf? Why can't we just have a normal elm datatype that represents a JSON value and write normal elm functions to process it?

Let's find out.

In this article, I'm going to try to lead you to designing elm's Json.Decode library. I'll assume you're comfortable writing elm and you know what JSON is in general, but that you don't know anything about how decoding works. By the end, I hope you'll understand how Json.Decode works and have an intuition for why it's designed the way it is.

JSON the normal way

Let's pretend that instead of Json.Decode.Value, we had a completely normal type defined as:

type JsonValue
    = String String
    | Int Int
    | Float Float
    | Bool Bool
    | Null
    | Obj (Dict String JsonValue)
    | Arr (List JsonValue)

A JSON value is a string, a number (Int or Float), a boolean, the value null, an object consisting of strings associated with other JSON values, or an array of JSON values. This is how you might expect JSON to be exposed in elm, and in any event it's totally bog-standard elm code with nothing fancy going on. Easy peasy.

Let's say we're writing a game client, and the server sends us groups of updates periodically in the form of a JSON message that looks like this:

{
  "operations": [
    {
      "action": "createEnemy",
      "name": "Zombie",
      "hitPoints": 4
    }, {
      "action": "movePlayer",
      "location": "forest"
    }
  ]
}

We need to use these in elm somehow. We could use the JsonValue representation directly, but that seems like a bad idea since it'd mean we'd have to deal with the details of the JSON format all through our program. Instead we should define a more reasonable elm type like this:

type Operation
    = CreateEnemy { name : String, hitPoints : Int }
    | MovePlayer { location : String }

and write a function that translates a JsonValue and produces a list of into a list of Operations:

readOperations : JsonValue -> List Operation

Oh, and we also need to account for what happens if we get a JSON value that isn't a legal operation. We could return a Maybe (List Operation), but for debugging we're probably going to want to know why the JSON we got wasn't an operation, so let's make it a Result String (List Operation) instead. So we want to write:

readOperations : JsonValue -> Result String (List Operation)

Since we're talking about JsonValues, this is just a normal elm function, we can write it out using just cases and normal elm things. Below is a first stab. (I'm going to show the whole program, but it's big and I'll talk about the important parts directly, so don't feel like you need to study it.)

readOperations : JsonValue -> Result String (List Operation)
readOperations jsonValue =
    case jsonValue of
        Obj obj ->
            case Dict.get "operations" obj of
                Just opsList ->
                    readOperationsList opsList

                Nothing ->
                    Err ("Expected an object with field \"operations\", got: " ++ toString jsonValue)

        _ ->
            Err ("Expected an object, got " ++ toString jsonValue)


readOperationsList : JsonValue -> Result String (List Operation)
readOperationsList jsonValue =
    case jsonValue of
        Arr arr ->
            collapseResults (List.map readOperation arr)

        _ ->
            Err ("Expected an array, got " ++ toString jsonValue)


collapseResults : List (Result String a) -> Result String (List a)
collapseResults results =
    let
        collapseResultsAcc accumulatedList results =
            case results of
                [] ->
                    Ok (List.reverse accumulatedList)

                first :: rest ->
                    case first of
                        Ok value ->
                            collapseResultsAcc (value :: accumulatedList) rest

                        Err str ->
                            Err str
    in
        collapseResultsAcc [] results


readOperation : JsonValue -> Result String Operation
readOperation jsonValue =
    case jsonValue of
        Obj fields ->
            case Dict.get "action" fields of
                Just (String "createEnemy") ->
                    readCreateEnemy fields

                Just (String "movePlayer") ->
                    readMovePlayer fields

                Just (String s) ->
                    Err ("Got invalid action: " ++ s)

                Just v ->
                    Err ("Expected a string, got: " ++ toString v)

                Nothing ->
                    Err ("Expected an object with field \"action\", got: " ++ toString jsonValue)

        _ ->
            Err ("Expected an object, got: " ++ toString jsonValue)


readCreateEnemy : Dict String JsonValue -> Result String Operation
readCreateEnemy fields =
    case Dict.get "name" fields of
        Just (String name) ->
            case Dict.get "hitPoints" fields of
                Just (Int hitPoints) ->
                    Ok (CreateEnemy { name = name, hitPoints = hitPoints })

                Just v ->
                    Err ("Expected an integer, got: " ++ toString v)

                Nothing ->
                    Err "Expected an object with field \"hitPoints\""

        Just v ->
            Err ("Expected a string, got: " ++ toString v)

        Nothing ->
            Err "Expected an object with field \"name\""


readMovePlayer : Dict String JsonValue -> Result String Operation
readMovePlayer fields =
    case Dict.get "location" fields of
        Just (String location) ->
            Ok (MovePlayer { location = location })

        Just v ->
            Err ("Expected a string, got: " ++ toString v)

        Nothing ->
            Err "Expected an object with field \"location\""

Holy cow, this function is huge! Somehow I needed nearly 100 lines of code just to read a pretty simple bit of JSON. And if the format were more complicated, this code would keep getting bigger.

What's going on here?

If you look the program over, you can see why. When we read from JSON, every step of the way there are things that could go wrong, and elm's type system (rightly) forces us to handle every possible error. That leads to a lot of boilerplate.

For instance, in readCreateEnemy, the first case of the first case handles the good path, and the entire rest of the function -- four more cases -- mechanically address all the ways reading could go wrong. This not only makes the code tedious to write, it also makes it tedious to read.

The problem is that the straightforward program we've written combines the details of two different concerns:

• What we should do in the "good case" to turn our JsonValue into an Operation assuming the JSON has the shape we expect. This concern is specific to our particular program and the way we want to translate our expected JSON format to our program-specific Operation type.
• How to detect if the input JSON doesn't have the expected shape and if so how to construct an appropriate error. This concern is not specific to anything in our program. It seems likely that almost any program we write that involves parsing JSON is going to have some kind of expected shape and will want to signal an error if the input doesn't conform to it.

The second concern is legitimate, of course -- we need to deal with the fact that our input JSON might not have the shape we expect. But that concern dominates so much of the code, and is so generic and boilerplate-ey, that it makes it hard to understand what our decoder does other than find format errors. So let's see if we can find a way to separate out the code for our two concerns so that we can put the error handling code into its own library.

Pulling out all that error handling code

Notice how every function starts with a case statement that checks that the JSON object is the appropriate tag, and returns an error message otherwise? Let's write some helper functions that just do that. Let's start with the primitive values that have straightforward elm equivalents:

{-| Reads a JSON string to a String. Returns an error if the input isn't a string.
-}
string : JsonValue -> Result String String
string jsonValue =
    case jsonValue of
        String s ->
            Ok s

        _ ->
            Err ("Expected a string, got " ++ toString jsonValue)


{-| Reads a JSON int to an Int. Returns an error if the input isn't an integer.
-}
int : JsonValue -> Result String Int
int jsonValue =
    case jsonValue of
        Int i ->
            Ok i

        _ ->
            Err ("Expected an integer, got " ++ toString jsonValue)

-- ... and similar for float and bool

Before we move on, one observation: We're writing the type JsonValue -> Result String [something] a lot to represent things that read a JSON value to some elm type. When I see a common pattern in types like this, I like to give it a name to help me think about it:

{-| Simple type alias for functions that parse a JsonValue into a value of
some arbitrary type t. Since the parse may fail, the function returns a
Result that could indicate a parse error.
-}
type alias JsonDecoder t = JsonValue -> Result String t

From now on I'll use this type. But when you see it, remember that it's just a function that reads a JsonValue and produces a result. We're still in the world of plain old elm functions and data structures.

Arrays

Now we've gotten rid of all that error handling boilerplate for all the flat JSON values, but we couldn't handle many interesting JSON values without also dealing with arrays and objects. Up to this point, the functions we've been writing only needed to do one thing: look at one level of JSON structure and either return the equivalent elm value or an error. But arrays and objects are containers for more JSON values that the caller probably has an opinion about: we don't just want a list, we want a list of strings in one place and maybe a list of integers in another.

Fortunately, elm makes it easy to snap functions together to build bigger functions, so let's make our array decoder take another decoder that it will use to decode each element. We can just use the implementation of readOperationsList we wrote earlier and modify it to take a decoder argument:

{-| Decodes a JSON array, with each element decoded by the given decoder.
-}
list : JsonDecoder a -> JsonDecoder (List a)
list elementDecoder jsonValue =
    case jsonValue of
        Arr jsonValues ->
            collapseResults (List.map elementDecoder jsonValues)
        _ ->
            Err ("Expected an array, got " ++ toString jsonValue)


{-| Groups a list of Results together into a result that either produces all the
successes in a list if all are Ok, or the first error if there are any.
-}
collapseResults : List (Result String t) -> Result String (List t)
collapseResults = ... -- see above for implementation

That's it!

Objects

Objects pose some problems of their own.

• For one thing, JSON objects have fields, and when we process an object we almost always want to get a specific named field and process that.
• For another, when we read an object like

{
  "action": "createEnemy",
  "name": "Zombie",
  "hitPoints": 4
}

we need to read multiple fields off of it and provide them to some other function to get a result -- in this case we need to read the name and hitPoints fields and provide them to CreateEnemy.

• Finally, we may need to figure out what to expect from an object by reading other pieces of it. For our operations, we need to read the "action" field to know whether we're making a CreateEnemy or a MovePlayer action and what other fields to expect to see on the object.

Let's tackle these one at a time.

Reading fields

Now that we've dealt with arrays, this seems pretty straightforward. We can do what we did there: write a function that takes a JsonDecoder and a field name as arguments, and returns a new JsonDecoder that expects to see an object that contains the given field and reads it with the given decoder. Here's how that looks:

{-| Decodes the named field of the given JSON object using the given decoder.
Returns an error if the given value isn't a JSON object.
-}
field : String -> JsonDecoder t -> JsonDecoder t
field name decoder jsonValue =
    case jsonValue of
        Obj dict ->
            case Dict.get name dict of
                Just fieldValue ->
                    decoder fieldValue

                Nothing ->
                    Err ("Expected an object with field \"" ++ name ++ "\", got: " ++ toString jsonValue)

        _ ->
            Err ("Expected an object, got: " ++ toString jsonValue)

Combining multiple fields

In our first version of decodeOperations, the function readCreateEnemy is responsible for reading the name and hitPoints fields of a JSON object and using the contents to build a CreateEnemy operation. It is easily the nastiest function in that implementation due to how much error handling we need to do, so if we're trying to clean up the error-handling code we're going to have to do something here. But it's not immediately apparent what.

Let's think about what we're going to need to do. First of all, readCreateEnemy's job is to build a CreateEnemy, a value that has nothing to do with JSON; it seems like a good idea to let the caller provide a function that does the combining while the boilerplate we're writing now handles calling it under the right circumstances. Since our job is to handle the errors, that function should just take the successful results of decoding the subfields, so we're also going to need the caller to tell us what subfields they want and how to decode their contents.

Let's write that out (pretending for the sake of argument we always want to read and combine exactly two fields):

potentialMultifieldDecoder :
    (a -> b -> t)
    -> String
    -> JsonDecoder a
    -> String
    -> JsonDecoder b
    -> JsonDecoder t

This seems like a function we could implement, but it's frustrating that we just wrote a function that decodes a field, and we're going to have to write it again in the body of this new function. This is one of these nice situations where we can do less work and make our library more powerful at the same time -- instead of forcing the caller to read fields from an object, let's allow them to decode anything they want! They can easily decode multiple fields from an object using the field function we just wrote, or they can decode and combine anything else they want.

That function becomes:

{-| Returns a decoder that returns the result of applying the given
function to the successful result of decoding using both of the given
decoders.
-}
object2 : (a -> b -> t) -> JsonDecoder a -> JsonDecoder b -> JsonDecoder t
object2 f aDecoder bDecoder jsonValue =
    case ( aDecoder jsonValue, bDecoder jsonValue ) of
        ( Ok a, Ok b ) ->
            Ok (f a b)

        ( Err s, _ ) ->
            Err s

        ( _, Err s ) ->
            Err s

We can also define object3, object4, and so on.

Looking at what we've written, does the type look familiar to you? If you read a lot of elm library code, you might have noticed that it fits the map pattern that shows up in a lot of libraries: for instance,

Maybe.map2 : (a -> b -> t) -> Maybe a -> Maybe b -> Maybe t
Result.map2 : (a -> b -> t) -> Result x a -> Result x b -> Result x t
Task.map2 : (a -> b -> t) -> Task x a -> Task x b -> Task x t

In all those libraries, a "map" function does the same kind of thing. We have some type like Maybe that we can think of as a "sort-of a" type -- Maybe a is an a that might not be there, Result x a is an a that might be an error value of type x instead, and Task x a is a recipe for an asynchronous task that might produce an a if we execute it. That makes map2 a "sort-of function application" given that we have a sort-of a and a sort-of b already, apply the function to the underlying a and b values if and once they exist. Since they're only sort of values, the result is also only sort of a value.

In our case, JsonDecoder a represents a "sort-of a" -- an a that we might get by reading it out of a JSON value. It fits the pattern nicely! In that context, it's clear that object2 is really just map2, and we should call it that for consistency with other elm libraries. Also, it makes it obvious that there's a good reason for us to add a map (i.e., map1) that transforms just a single argument. We can use this for more than just reading fields off of an object!

map2 : (a -> b -> t) -> JsonDecoder a -> JsonDecoder b -> JsonDecoder t
map2 = object2


{-| Returns a decoder that returns the result of applying the given
function to the successful result of decoding using the given decoder.
-}
map : (a -> t) -> JsonDecoder a -> JsonDecoder t
map f decoder jsonValue =
    decoder jsonValue
        |> Result.map f

Making decisions while decoding

At this point we've built up a library that factors out error handling nicely for almost everything we did in our original program. The one problem we have left is our original readOperation function, which looks at the action field and decides whether to call readCreateEnemy or readMovePlayer. If we think of readOperation as being composed of the "real function" and the "error boilerplate," then the real function is the part that reads the action field as a string and then performs case dispatch on it to call either readCreateEnemy or readMovePlayer, and the error boilerplate is the series of four cases at the end of the function that handle the scenarios:

1. When the JSON object we're reading specifies an action field that isn't "createEnemy" or "movePlayer",
2. When it specifies an action field that isn't a string,
3. When it doesn't have an action field, and
4. When it isn't an object at all.

The first one is pretty specific to reading Operations, but the others are completely generic: in fact, field "action" string already handles all three. So let's figure out a way to reuse that. We need the user to specify the "real function" from above, so let's just take it directly as a (real) function. This function should take a successfully-decoded string, but what should it return? In the code we're trying to remove boilerplate from it returned readCreateEnemy and readMovePlayer, both of which can be thought of as JsonDecoders. So our boilerplate could take a JsonDecoder a and a function a -> JsonDecoder b and should return a JsonDecoder b. Here's that type all at once:

(a -> JsonDecoder b) -> JsonDecoder a -> JsonDecoder b

That type looks familiar too! It shows up in lots of elm packages under the name andThen: for instance, just in core, we've got

Maybe.andThen : (a -> Maybe b) -> Maybe a -> Maybe b
Result.andThen : (a -> Result x b) -> Result x a -> Result x b
Task.andThen : (a -> Task x b) -> Task x a -> Task x b

and on and on.

In all those cases, andThen does basically the same thing we want it to do here. Remember map represents applying a function a -> b to a "sort-of a"; we might not not get our a, but if we do we can definitely convert it to a b. andThen is for the situation where we have a sort-of a, and once we actually get our hands on an a we want to use it to figure out how to make a sort-of b. For instance, you can use

Maybe.map (\x -> x + 1) maybeNumber

to add 1 to maybeNumber if it exists, and

Maybe.andThen (\x -> if x >= 5 then (Just x) else Nothing) maybeNumber

to turn maybeNumber into Nothing if it's less than 5.

This pattern is exactly what we want to do with decoders, so let's call our function andThen. It's easy enough to implement:

{-| Returns a decoder that runs the given decoder against its input. Then,
if the decode is successful, it applies the given function and re-parses
the input JSON against the resulting second decoder. This allows a decoder
that reads part of the JSON object before deciding how to parse the rest
of the object.
-}
andThen : (a -> JsonDecoder b) -> JsonDecoder a -> JsonDecoder b
andThen toB aDecoder jsonValue =
    case aDecoder jsonValue of
        Ok a ->
            (toB a) jsonValue

        Err s ->
            Err s

One last detail: Our boilerplate handles the generic problems, but using andThen we might find ourselves writing logic that wants to directly return a success with a particular value or a failure without running any more child decoders. We can't just return Ok or Error values in those cases because andThen wants us to return a JsonDecoder -- which, remember, is a function that takes a JsonValue to a Result, not a Result itself. But we can do the next best thing:

{-| Returns a decoder that always succeeds with the given value.
-}
succeed : a -> JsonDecoder a
succeed a jsonValue =
    Ok a


{-| Returns a decoder that always fails with the given error message.
-}
fail : String -> JsonDecoder a
fail str jsonValue =
    Err str

Parsing operations, take two

Now we've written everything we need to pull out all the boilerplate from readOperation. Let's see what it looks like when we take all that boilerplate out:

readOperations : JsonDecoder (List Operation)
readOperations =
    JsonDecoder.list <|
        JsonDecoder.field "action" JsonDecoder.string
            |> JsonDecoder.andThen
                (\action ->
                    case action of
                        "createEnemy" ->
                            JsonDecoder.map2
                                (\name hitPoints ->
                                    CreateEnemy
                                        { name = name
                                        , hitPoints = hitPoints
                                        }
                                )
                                (JsonDecoder.field "name" JsonDecoder.string)
                                (JsonDecoder.field "hitPoints" JsonDecoder.int)

                        "movePlayer" ->
                            JsonDecoder.map
                                (\location -> MovePlayer { location = location })
                                (JsonDecoder.field "location" JsonDecoder.string)

                        _ ->
                            JsonDecoder.fail ("Got invalid action: " ++ action)
                )

Wow! This version is only about a quarter of the size of our first attempt, and it's much more readable. If you squint a bit, it reads almost like a straightforward description of the JSON format: It's a list of objects with field called action. If action is "createEnemy" then it should have name and hitPoints fields, if it's "movePlayer" then it should have a location field, and anything else is illegal. I would much prefer to maintain this version of the parse than what we started with.

It's worth reflecting on what we've done here. The only thing we did was try to abstract away the boilerplate involved in handling the cases where we're trying to decode a JSON value and discover it doesn't have the shape we expected. There's no magic involved here, everything was just short, relatively straightforward code, and yet we were able to transform an unreadable mess into something that basically makes sense, and in the process we got a nice library that we could use for other projects.

Congratulations! You designed Json.Decode!

The bad news is, the Json.Decode library doesn't have a JsonValue type like the one we've been using here. It's implemented in native Javascript and doesn't give you access to the underlying data structures it uses.

The good news is, every other function we've written here was actually taken straight from the Json.Decode API: replace JsonDecoder with Decoder and everything works just the same! And as I hope I've convinced you, even Json.Decode did give you a JsonValue equivalent, you'd want to use the API functions anyway. Now go read the rest of the API: everything there is just an extension of the ideas we've worked out here. I hope that now that you've figured out from first principles why the library works the way it does, you'll find it a little less mysterious and easier to work with.