Command line tool to auto-generate custom types for Elm with accompanying JSON encoders/decoders from a file of enum definitions.
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README.md

elm-enums automatic code generator

Summary: elm-enums is a command-line tool to autogenerate Elm type declarations and JSON encoders/decoders for simple enum-like custom types. If you're just interested in how to use it then you can jump straight to the How It Works section below.

The Problem (a.k.a. elm-enums' Raison d'être)

Often you have a finite list of possible values that something can take. While it's perfectly possible to represent such values simply as a String, often we prefer to make a very simple custom type to deal with this, e.g.

type Animal
    = Cat
    | Dog
    | Bird
    | Rat

The advantage of this approach is that the compiler will stop any errors creeping into our code if/when we make a typo in later referring to one of the values: something that otherwise can be quite annoying to track down. It also forces us to consider all possibilities when making a decision based on the value (unless we cheat and use _ -> in case statements!).

This advantage becomes more important the more we have to pass such types around. Consider the case of an interactive story. The player moves from chapter to chapter in the story depending on their actions; some chapters may be revisited or even change their appearance depending on what accomplishments the player has already made and what items they have acquired. If we were to try and code this in Elm we would need to keep track of which chapters a player had already visited, which chapter they were on now, what objects they were carrying and what objectives they had completed. The rules for transitioning from chapter to chapter would depend on all these things. At this point referring to chapters simply by a name encoded as a String becomes very error-prone. Typos can cause references to non-existent chapters, and it's very easy to forget to consider all possibilities when writing logic to determine what happens to a player based on all the things they could already have done.

Having custom types to refer to the chapters, items and accomplishments becomes very appealing:

type ChapterName
    = Introduction
    | Meeting1
    | Meeting2
    | AChoiceOfPaths
    | FollowingTheRedPath
    | FollowingTheGreenPath
    | ...

type Accomplishment
    = FixedTheCar
    | FoundTheMap
    | ...

...

we can then encode the logic of how to render the current section of story to the user in a function that takes these types, e.g.

viewStory : ChapterName -> List ChapterName -> List Accomplishments -> List Item -> Html Msg
viewStory currentChapter exploredChapters accomplishments items =
   ...

and the compiler will ensure that we can't refer to non-existent chapters and that we don't forget to consider any possibility when determining what options the chapter should offer to the player based on what they have with them and what they have already accomplished.

This all works wonderfully until things get so large that we want persistence: to somehow save the player's state so they can return to the game at another time. As soon as this happens we find we need to encode our nice custom types in a way that javascript or a server can handle and store for us. So now we need to write JSON encoders and decoders for all our types. Not only does this take a bit of time to initially set up, but more importantly while it might still help with keeping our encoder up to date, the compiler no longer has our back when it comes to keeping the decoder up to date. Suddenly adding a new chapter and a bunch of new accompanying accomplishments requires us not only to add them to our custom types, but also to our encoders and decoders; and if we forget to add one of them to the decoder (or if we make a typo when doing so in either the decoder or the encoder) then suddenly our game can encounter saved state being handed back to it that it can't understand.

The Solution

elm-enums is a very basic command-line tool (written in Elm!) to take the pain out of handling simple custom types (of the form that other languages would refer to as enums) that need passing in and out of JSON and are prone to frequent change in their set of values. It can't handle more complex custom types at all (i.e. anything that is more than just a union of simple argumentless constructors), but it aims to make those it can handle as maintenance-free as possible.

How it works

To use elm-enums you create a single input file called enums.defs in your source tree. In it you place all the enum-like type definitions you want elm-enums to handle for you. Each definition must start with the enum keyword and follow the following pattern:

{- Chapter names

By convention we prefix all chapters that cause a game end with "Final".
-}
enum ChapterName =
    [ Introduction -- the game always starts here
    , Meeting1
    , Meeting2
    , AChoiceOfPaths
    , FollowingTheRedPath
    , FollowingTheGreenPath
    , FinalAStickyEnd
    , FinalSmallVictory
    ]

note that Elm-like comments are supported. Whitespace is totally ignore so you are free to format/indent the definitions however you like.

The name assigned to a enum and all its declared values can only contain alphanumeric ASCII characters and the underscore _ chararacter (note this is more restrictive than Elm itself, additional characters may be allowed in later versions). Additionally names and values must be unique, begin with an uppercase letter, and cannot match the names of any of the Elm default imports.

If you run elm-enums without any arguments from the same directory as your enums.defs it will then parse this file and automatically generate a file Enums.elm in the same directory, which will contain custom types and JSON encoders/decoders for all your enum definitions. To use these from any or your Elm modules, simply import what you need using the following pattern:

import Enums exposing (ChapterName(..), encodeChapterName, decodeChapterName, Item(..), encodeItem, decodeItem)

All custom types will have the name given in your enum declaration and encoders/decoders will have the same name but prefixed with encoder/decoder appropriately. The type signature of the encoders/decoders is always:

decodeMyEnum : Json.Decode.Decoder MyEnum

encodeMyEnum : MyEnum -> Json.Encode.Value

If you need to update an enum simply modify enums.defs and then rerun elm-enums. The produced Enums.elm is preformatted to the standards of elm-format, both so that it is easy to read and so that no change will occur to the file if you have any git hooks to run elm-format on everything before comitting.

Current Limitations

As discussed above, elm-enums currently needs to be run from the same directory that your enums.defs is in and currently only ASCII alphanumeric characters (plus the underscore) are accepted in names and values.