Elminator
Generate Elm type definitions and JSON encoders/decoders from Haskell source (for Elm 0.19 and 0.18)
- Supports generation of polymorphic types (as well as concrete ones) in Elm from possibly polymorphic Haskell types, including types with phantom type variables.
- Supports generation of recursively defined types.
- Generates code that does not depend on external Elm libraries.
- Does not have limits on the number of fields that the constructors of your type can have.
- Supports JSON encoding options exported by the Aeson library comprehensively (The tests exhaustively check the Elm/Haskell round tripping of values for all possible configurations of Aeson.options)
- 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.
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.
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)
data GenOption
= Definiton PolyConfig
| EncoderDecoder
| Everything PolyConfig
data PolyConfig
= Mono | Poly
A sample of generated Elm code can be seen here
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
.
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])
Tests
This is being tested by round tripping a bunch of JSON encoded values from an Elm front end to a Haskell back end, where it is decoded and sent back to Elm where it is again decoded and checked for equality with the value that was initially sent. These right now, are in the form of a quick hack Python script that makes the Haskell builds and auto generated Elm source for all possible values of Aeson.Options and testing the round tripping of included types using a headless Chromium browser. The test repo is separate for now and is available at https://bitbucket.org/sras/elminator-test.
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).
extra-deps:
elminator-0.2.1.0