# Elminator Generate Elm type definitions and JSON encoders/decoders from Haskell source (for Elm 0.19 and 0.18) 1. Supports generation of polymorphic types (as well as concrete ones) in Elm from possibly polymorphic Haskell types, including types with phantom type variables. 2. Supports generation of direct and indirect recursive types as long as the recursive types are not polymorphic. 3. Generates code that does not depend on external Elm libraries. 4. Does not have limits on the number of fields that the constructors of your type can have. 5. Supports JSON encoding options exported by the Aeson library comprehensively. 6. Supports generation of code that depend on user defined types and encoders/decoders in Elm. ### How to use? To generate Elm code for a Haskell type, the Haskell type needs to have an instance of the `ToHType` type class. This can be automatically derived, provided all your constructor field types have `ToHType` instances. A sample can be seen below. Please note that language extensions `DeriveGeneric` and `DeriveAnyClass` should be enabled to make this work. ```haskell {-# Language DeriveGeneric #-} {-# Language DeriveAnyClass #-} module Lib where import Elminator import GHC.Generics (Generic) data SingleCon = SingleCon Int String deriving (Generic, ToHType) ``` Since this library uses template Haskell to look up type information (in addition to Generics), we need to run the code generation code in a template Haskell splice. A usage sample can be seen in the following code used in the round trip tests for this library. ```haskell {-# Language OverloadedStrings #-} {-# Language TemplateHaskell #-} module CodeGen where import Data.Proxy import Elminator import Data.Text.IO import Data.Text import Lib elmSource :: Text elmSource = $(generateFor Elm0p19 myDefaultOptions (Just "./elm-app/src/Autogen.elm") $ do include (Proxy :: Proxy SingleCon) $ Everything Mono include (Proxy :: Proxy SingleRecCon) $ Everything Mono include (Proxy :: Proxy SingleConOneField) $ Everything Mono include (Proxy :: Proxy SingleRecConOneField) $ Everything Mono include (Proxy :: Proxy TwoCons) $ Everything Mono include (Proxy :: Proxy TwoRecCons) $ Everything Mono include (Proxy :: Proxy BigCon) $ Everything Mono include (Proxy :: Proxy BigRecCon) $ Everything Mono include (Proxy :: Proxy MixedCons) $ Everything Mono include (Proxy :: Proxy Comment) $ Everything Mono include (Proxy :: Proxy WithMaybes) $ Everything Mono include (Proxy :: Proxy WithSimpleMaybes) $ Everything Mono include (Proxy :: Proxy (WithMaybesPoly (Maybe String) Float)) $ Definiton Poly include (Proxy :: Proxy (WithMaybesPoly (Maybe String) Float)) EncoderDecoder include (Proxy :: Proxy (Phantom ())) $ Everything Poly include (Proxy :: Proxy (TypeWithPhantom Float)) $ Everything Poly include (Proxy :: Proxy RecWithList) $ Everything Mono include (Proxy :: Proxy IndRecStart) $ Everything Mono include (Proxy :: Proxy IndRec2) $ Everything Mono include (Proxy :: Proxy IndRec3) $ Everything Mono include (Proxy :: Proxy NTSingleCon) $ Everything Mono include (Proxy :: Proxy NTSingleCon2) $ Everything Poly include (Proxy :: Proxy Tuples) $ Everything Mono include (Proxy :: Proxy NestedTuples) $ Everything Mono include (Proxy :: Proxy (NestedTuplesPoly ())) $ Definiton Poly include (Proxy :: Proxy (TypeWithExt ())) $ Everything Poly include (Proxy :: Proxy (WithEmptyTuple ())) $ Everything Poly include (Proxy :: Proxy (Phantom2 ())) $ Everything Poly include (Proxy :: Proxy PhantomWrapper) $ Everything Poly) -- The `generateFor` function accepts an elm version (Elm0p19 or Elm0p18), a value of type `Options` from the Aeson library -- , and optional `FilePath` to which the generated source will be written to, and a `Builder` value. -- The `Builder` is just a `State` monad that aggregates the configuration parameters from the include -- calls. The first parameter of the include function is a `proxy` value that denotes the type that requires Elm code generation. -- The second value is a value of type `GenOption` that selects which entities needs to be generation, and also selects if the -- type generated at Elm should be polymorphic. It is defined as follows. data GenOption = Definiton PolyConfig -- Generate Type definition in Elm. PolyConfig field decides if the type has to be polymorphic | EncoderDecoder -- Generate Encoder and Decoder in Elm | Everything PolyConfig -- Generate both type definition, encoders and decoders. PolyConfig field decides if the type has to be polymorphic. data PolyConfig = Mono | Poly ``` The Elm code generated by the above code can be seen [here](https://bitbucket.org/sras/elminator-test/src/master/elm-app/src/Autogen.elm) ### How to depend on predefined types and encoders/decoders This is intended to be an escape hatch in cases where the types you want to generate elm code for, in turn contains types that you didn't define, and do not have access to the internals of. This feature basically allows you to define the Elm type and encoders/decoders yourself, and let the generated code import it and use them in the generated code. To use this, derive the `ToHType` instance for the type using the `HExternal` constructor of the `HType` type. Sample code can be seen below, where we define a `ToHType` instance for a type called `MyExtType`. ```haskell instance (ToHType a, ToHType b) => ToHType (MyExtType a b) where toHType _ = do ha <- toHType (Proxy :: Proxy a) hb <- toHType (Proxy :: Proxy b) pure $ HExternal (ExInfo ("External", "MyExtType") (Just ("External", "encodeMyExtType")) (Just ("External", "decodeMyExtType")) [ha, hb]) ``` ### Installing If you are using the Stack tool, then for the time being, you have to add Elminator to the 'extra-deps' section of stack.yaml as follows (Please use the latest available version here). ```yaml extra-deps: elminator-0.2.1.0 ```