gen-elm-wrappers

You are writing an Elm program. You have a custom type, which you would like to use for the keys of a Dict, or the elements of a Set. Unfortunately, for both of these cases, your type has to be comparable, and custom types cannot be comparable. What to do?

Solution: add a gen-elm-wrappers.json, run gen-elm-wrappers, and it will generate one or more Elm modules that wrap Dict or Set, so that you can use them with your custom type.

Installing

If you’re using node anyway, you can download it from NPM: npm install --save-dev gen-elm-wrappers or install it globally or using yarn/pnpm/whatever. This won’t install it on your $PATH unless your $PATH already includes ./node_modules/.bin. You might only want to run it via a script in your package.json anyway.

Otherwise, you can download prebuilt binaries. Each file is a self-contained executable for the appropriate platform. Rename it to gen-elm-wrappers and move it to somewhere on your $PATH. chmod +x it if you’re not on Windows.

Or to install from source:

• brew install go (or whatever the best way of installing Go on your laptop is)
  • don’t worry, you don’t need to know Go to use this
• npm ci
• npm test

Assuming the tests all pass, this builds a gen-elm-wrappers executable. Copy it to somewhere on your $PATH.

Using

Create a gen-elm-wrappers.json file. This needs to contain an object which (will eventually contain more config data, but currently only) contains a generate key holding an array of module definitions.

To wrap Dict, the module definition is an object containing these keys:

• underlying-type
  • Must be "Dict"
• wrapper-type
  • The fully-qualified name of the type to generate. The generated code will be stored in the module named here. e.g. to generate a Foo.Bar module containing a MyDict type, you would put "Foo.Bar.MyDict" here
• public-key-type
  • The fully-qualified name of your custom type that you want to use as keys
• private-key-type
  • The type of keys to use in the underlying Dict. This will typically be Int or String, but can be any concrete comparable type
• public-key-to-private-key
  • The fully-qualified name of a function that converts values from public-key-type to private-key-type. i.e. it has a type like PublicKey -> PrivateKey
• private-key-to-public-key
  • The fully-qualified name of a function that converts values from private-key-type to public-key-type. It has a type like PrivateKey -> Maybe PublicKey. You can’t use a function with a type like PrivateKey -> PublicKey here — you may need to write a wrapper function in your code with the correct type

To wrap Set, the module definition is an object containing these keys:

• underlying-type
  • Must be "Set"
• wrapper-type
  • The fully-qualified name of the type to generate. The generated code will be stored in the module named here. e.g. to generate a Foo.Bar module containing a MySet type, you would put "Foo.Bar.MySet" here
• public-key-type
  • The fully-qualified name of your custom type that you want to use as elements in the set
• private-key-type
  • The type of elements to use in the underlying Set. This will typically be Int or String, but can be any concrete comparable type
• public-key-to-private-key
  • The fully-qualified name of a function that converts values from public-key-type to private-key-type. i.e. it has a type like PublicKey -> PrivateKey
• private-key-to-public-key
  • The fully-qualified name of a function that converts values from private-key-type to public-key-type. It has a type like PrivateKey -> Maybe PublicKey. You can’t use a function with a type like PrivateKey -> PublicKey here — you may need to write a wrapper function in your code with the correct type

Then, run gen-elm-wrappers. It expects elm.json to be in the current directory. It writes the generated code to the appropriate location inside your src directory.

For Dicts, the generated code wraps all functions from the core Dict module. If your program also has elm-community/dict-extra as a direct dependency, it will also wrap several functions from Dict.Extra.

For Sets, the generated code wraps all functions from the core Set module. If your program also has stoeffel/set-extra as a direct dependency, it will also wrap some functions from Set.Extra.

If elm-format is on your PATH (and not a relative path, i.e. not starting with . or ..) then the generated code will be beautifully formatted. (This is the case, for example, when elm-format and gen-elm-wrappers were both installed locally using npm, and you’re running gen-elm-wrappers via npm.)

Example

If you put this in src/Helpers.elm:

module Helpers exposing (..)

import Time

maybePosixFromMillis : Int -> Maybe Time.Posix
maybePosixFromMillis millis =
    Just <| Time.millisToPosix millis

and then you put this in gen-elm-wrappers.json:

{
    "generate": [
        {
            "underlying-type": "Dict",
            "wrapper-type": "Type.DictTimePosix.DictTimePosix",
            "public-key-type": "Time.Posix",
            "private-key-type": "Int",
            "private-key-to-public-key": "Helpers.maybePosixFromMillis",
            "public-key-to-private-key": "Time.posixToMillis"
        },
        {
            "underlying-type": "Set",
            "wrapper-type": "Type.SetTimePosix.SetTimePosix",
            "public-key-type": "Time.Posix",
            "private-key-type": "Int",
            "private-key-to-public-key": "Helpers.maybePosixFromMillis",
            "public-key-to-private-key": "Time.posixToMillis"
        }
    ]
}

then gen-elm-wrappers will produce

• a Type.DictTimePosix module in src/Type/DictTimePosix.elm, containing a DictTimePosix v type that acts like a Dict with Time.Posix keys and v values
• a Type.SetTimePosix module in src/Type/SetTimePosix.elm, containing a SetTimePosix type that acts like a Set with Time.Posix elements

Limitations

• It won’t work if you try to wrap more than one type into the same module
• Set.map is not wrapped
• Dict.Extra:
  • removeMany and keepOnly are not wrapped, even when Set is also being wrapped for the key type
  • mapKeys and invert are not wrapped
• Set.Extra:
  • concatMap and filterMap are not wrapped

Portability

I’ve only tested this on my Mac. But it’s written in Go, and I hear Go’s really portable, so presumably it should also run on *BSD, various Linuxes, Windows, smart toasters, ZX-81, etc.

Actually… the test script won’t run on Windows. (Unless you use WSL?)

Roadmap

This isn’t in priority order yet and I’ve probably forgotten something.

• Support type variables in dict key types and set element types
• Support versions of elm-community/dict-extra before 2.4.0
• Support versions of stoeffel/set-extra before 1.2.0
• Wrap more functions from elm-community/dict-extra and stoeffel/set-extra
• Support writing the generated code to a directory other than src; optionally wipe it first
• Write more unit tests around reading the config from elm.json
• Improve error messages when something’s wrong in elm.json
• Validate identifiers in the config (instead of blindly writing them out and letting Elm complain about them)

Development

If you don’t know Go: this is written in Go, sorry.

If you do know Go: I’m learning Go, this is my first Go program, the code’s probably highly non-idiomatic, sorry.

npm run test runs the unit tests; on success it then runs the component tests.

If you brew install fswatch then you can npm run test:go:watch. This runs the unit tests whenever the source code changes.

If you’re a hiring manager or a recruiter

I’m not looking for a job, no. Although I’m always interested in hearing about jobs involving Elm and/or climate tech.

• Senior / tech lead roles
• Full stack or backend
• Permanent only (no contracting)
• IC only (no line management)
• Remote (UK timezone ± an hour or two) or on-site/hybrid (London/Medway); not willing to relocate
• I prefer to work with statically typed languages (e.g. Typescript, not plain Javascript)
• not blockchain (except for catching crims), not ad tech (unless it’s surveillance-free), don’t really want to work for a hedge fund
• Ich kann ein bisschen Deutsch sprechen, aber mein Deutsch ist schlect und Englisch is meine Muttersprache.

Please connect to me on LinkedIn and mention this repo in your invitation (and don’t bury the lede if you want to talk to me about an Elm or climate tech job).