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 in TravisCI 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.
Hackage page: https://hackage.haskell.org/package/elminator
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.
{-# 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.
{-# 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 "Autogen" (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
-- , a module name for the generated module, and an 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
A sample of generated Elm code can be seen here
How to explicitly map a Haskell type to an Elm type
Say you have this type defined in Haskell
data Product = Product { pName :: String, pWeight :: Decimal }
We can derive ToHType
for the above type just fine. This is because we have this general ToHType instance that use the Typeable
instances to create primitive type representation.
instance {-# OVERLAPPABLE #-} (Typeable a) => ToHType a where
toHType p = pure $ mkHType p
Even though we are able to derive HType instance, the generated code end up looking something like the following
type Product = Product { pName : String, pWeight : DecimalRaw }
encodeProduct : Product -> E.Value
encodeProduct a =
case a of
Product x -> E.object ([ ("pName", E.string (x.pName))
, ("pWeight", encodeDecimalRaw (x.pWeight))])
decodeProduct : D.Decoder Product
decodeProduct =
D.oneOf ([ let
mkProduct a1 a2 =
Product ({pName = a1, pWeight = a2})
in D.map2 (mkProduct) (D.field ("pName") (D.string)) (D.field ("pWeight") (encodeDecimalRaw))])
But there is no DecimalRaw
type on the Elm side. So in this case, we might want to use Float
on Elm side whenever we have a Decimal
field in Haskell.
This can be done as follows
instance ToHType Decimal where
toHType _ = toHType (Proxy :: Proxy Float)
This gives us usable Elm code.
type Product = Product { pName : String, pWeight : Float }
encodeProduct : Product -> E.Value
encodeProduct a =
case a of
Product x -> E.object ([ ("pName", E.string (x.pName))
, ("pWeight", E.float (x.pWeight))])
decodeProduct : D.Decoder Product
decodeProduct =
D.oneOf ([ let
mkProduct a1 a2 =
Product ({pName = a1, pWeight = a2})
in D.map2 (mkProduct) (D.field ("pName") (D.string)) (D.field ("pWeight") (D.float))])
Note that this only works if both types have compatible JSON representations. The Aeson instances
should take care of this on the Haskell side.
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 Python script that walks through the full range of Aeson options, make the Haskell build and auto generated Elm source for each, and then test the round tripping of included types using a headless Chromium browser. The tests at TravisCI use this process as well. 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