elm-gen

Experimental CLI tool for generating Elm JSON Decoders and Encoders, written mostly in Elm. Based on top of elm-ast parsing library.

Usage

Online demo

Simplified demonstration available here.

CLI tool

CLI tool can be obtained from NPM. After installation, you can used it like that:
elm-gen decoders&encoders Model.elm .
Call above will produce new file ModelEncodersAndDecoders.elm, which will contain decoders and encoders for types present in Model.elm.
Also note, that generated elm code for decoders uses NoRedInk elm-decode-pipeline package functions, so it must be installed into your project.
For more examples you may also look at functional tests in ts/MainTests.ts and correspondent files in tests_data folder.

Features

• elm-gen can generate decoders and/or encoders for user defined Record and Union types without type variables.
• elm-gen supports basic elm types supported by Json.(Encode|Decode) plus tuples and Maybe's.
• elm-gen can use hand-made decoders and encoders Decoder X and X -> Value in order to generate decoders for types, dependent on type X
• elm-gen will follow explicit import type references to look for type definitions doesnot present in current file
• elm-gen dont require to break your Elm code in any way (its still compilable by elm-make without pre-processing), except for some meta-comments insertion.

Config

Some configuration (mostly about naming) can be applied from config json-file. Example of config can be found in tests_data folder. It's correspondent Elm type representation resides in Config.elm

Meta-Comments

Elm-gen supports meta-comments in source code, in form of either -- //Meta-Comment or {-| //Meta-Comment -}.
For now, there are 4 meta-comments:

• Ignore -- elm-gen completely ignores next type definition (literally skipping it during parse stage).

• DefaultValue -- use next pair of (value type def, value expr) to produce default value for Json.Decode.Pipeline.optional function. Supports values of types defaultValue: UnionType for union types, and defaultValue : Record or defaultValue : Record -> Record for record types.

• NoDeclaration -- elm-gen won't generates type declaration for next type's decoder (usefull for preserving record structural typing ability for generated decoder )

• Field name aliases - You can define aliases for some field names to avoid issues with elm reserved words, and to make some special field names (like _id and _rev which stored by Couchdb) normally looking in Elm representation. For example, I use meta-comment and record to define some common aliases for Couchdb-mapped types:

{-| //FieldNameMapping
-}
couchAliases =
    { id = "_id"
    , rev = "_rev"
    , type_ = "_type"
    }

And later I apply such mapping to other type via another meta-comment:

{-| //UseFieldNameMapping(couchAliases)
-}
type alias NewUser =
    { id : String
    , name : String
    , password : String
    , type_ : String
    , ...
    }