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README
elm-street
Crossing the road between Haskell and Elm.
What this library about?
Elm-street
allows you to generate automatically derived from Haskell types
definitions of Elm data types, JSON encoders and decoders. This helps to avoid
writing and maintaining huge chunk of boilerplate code when developing full-stack
applications.
Getting started
In order to use elm-street
features, you need to perform the following steps:
- Add
elm-street
to the dependencies of your Haskell package. - Derive the
Elm
typeclass for relevant data types. You also need to derive JSON instances according toelm-street
naming scheme. This can be done like this: ```haskell import Elm (Elm, elmStreetParseJson, elmStreetToJson)
data User = User { userName :: Text , userAge :: Int } deriving (Generic) deriving anyclass (Elm)
instance ToJSON User where toJSON = elmStreetToJson instance FromJSON User where parseJSON = elmStreetParseJson
> **NOTE:** This requires extensions `-XDerivingStrategies`, `-XDeriveGeneric`, `-XDeriveAnyClass`.
Alternatively you can use `-XDerivingVia` to remove some boilerplate (available since GHC 8.6.1):
```haskell
import Elm (Elm, ElmStreet (..))
data User = User
{ userName :: Text
, userAge :: Int
} deriving (Generic)
deriving (Elm, ToJSON, FromJSON) via ElmStreet User
- Create list of all types you want to expose to Elm:
haskell type Types = '[ User , Status ]
> NOTE: This requires extension-XDataKinds
. - Use
generateElm
function to output definitions to specified directory under specified module prefix.main :: IO () main = generateElm @Types $ defaultSettings "frontend/src" ["Core", "Generated"]
> NOTE: This requires extension-XTypeApplications
.
The above command when called generates the following files:
+ `frontend/src/Core/Generated/Types.elm`: `Core.Generated.Types` module with the definitions of all types
+ `frontend/src/Core/Generated/Encoder.elm`: `Core.Generated.Encoder` module with the JSON encoders for the types
+ `frontend/src/Core/Generated/Decoder.elm`: `Core.Generated.Decoder` module with the JSON decoders for the types
+ `frontend/src/Core/Generated/ElmStreet.elm`: `Core.Generated.ElmStreet` module with bundled helper functions
Elm-side preparations
If you want to use capabilities provided by elm-street
in your Elm
application, you need to have several Elm packages preinstalled in the project. You
can install them with the following commands:
elm install elm/time
elm install elm/json
elm install NoRedInk/elm-json-decode-pipeline
elm install rtfeldman/elm-iso8601-date-strings
Library restrictions
Elm-street
is not trying to be as general as possible and support every
use-case. The library is opinionated in some decisions and contains several
limitations, specifically:
- Record fields must be prefixed with the type name or its abbreviation. ```haskell data UserStatus = UserStatus { userStatusId :: Id , userStatusRemarks :: Text }
data HealthReading = HealthReading { hrUser :: User , hrDate :: UTCTime , hrWeight :: Double }
2. Data types with type variables are not supported (see [issue #45](https://github.com/Holmusk/elm-street/issues/45) for more details).
Though, if type variables are phantom, you still can implement `Elm` instance which
will generate valid Elm defintions. Here is how you can create `Elm` instance for
`newtype`s with phantom type variables:
```haskell
newtype Id a = Id { unId :: Text }
instance Elm (Id a) where
toElmDefinition _ = elmNewtype @Text "Id" "unId"
- Sum types with records are not supported (because it's a bad practice to have records in sum types). ```haskell -- - Not supported data Address = Post { postCode :: Text } | Full { fullStreet :: Text, fullHouse :: Int }
4. Sum types with more than 8 fields in at least one constructor are not
supported.
```haskell
-- - Not supported
data Foo
= Bar Int Text
| Baz Int Int Text Text Double Double Bool Bool Char
- Records with fields that reference the type itself are not supported. This
limitation is due to the fact that
elm-street
generatestype alias
for record data type. So the generated Elm type for the following Haskell data type won't compile in Elm:haskell data User = User { userName :: Text , userFollowers :: [User] }
- Generated JSON encoders and decoders are consistent with default behavior of
derived
ToJSON/FromJSON
instances from theaeson
library except you need to strip record field prefixes. Fortunately, this also can be done generically. You can use functions fromElm.Aeson
module to deriveJSON
instances from theaeson
package. - Only
UTCTime
Haskell data type is supported and it's translated toPosix
type in Elm. - Some words in Elm are considered reserved and naming a record field with one of these words (prefixed with the type name, see 1) will result in the generated Elm files to not compile. So, the following words should not be used as field names:
if
then
else
case
of
let
in
type
module
where
import
exposing
as
port
tag
(reserved for constructor name due toaeson
options)
- For newtypes
FromJSON
andToJSON
instances should be derived usingnewtype
strategy. AndElm
should be derived usinganyclass
strategy:haskell newtype Newtype = Newtype Int deriving newtype (FromJSON, ToJSON) deriving anyclass (Elm)
Play with frontend example
The frontend
directory contains example of minimal Elm project that shows how
generated types are used. To play with this project, do:
- Build and execute the
generate-elm
binary:cabal new-run generate-elm
- Run Haskell backend:
cabal new-run run-backend
- In separate terminal tab go to the
frontend
folder:cd frontend
- Run the frontend:
elm-app start
Generated examples
Below you can see some examples of how Haskell data types are converted to Elm
types with JSON encoders and decoders using the elm-street
library.
Records
Haskell
data User = User
{ userName :: Text
, userAge :: Int
} deriving (Generic)
deriving (Elm, ToJSON, FromJSON) via ElmStreet User
Elm
type alias User =
{ name : String
, age : Int
}
encodeUser : User -> Value
encodeUser x = E.object
[ ("name", E.string x.name)
, ("age", E.int x.age)
]
decodeUser : Decoder User
decodeUser = D.succeed User
|> required "name" D.string
|> required "age" D.int
Enums
Haskell
data RequestStatus
= Approved
| Rejected
| Reviewing
deriving (Generic)
deriving (Elm, ToJSON, FromJSON) via ElmStreet RequestStatus
Elm
type RequestStatus
= Approved
| Rejected
| Reviewing
showRequestStatus : RequestStatus -> String
showRequestStatus x = case x of
Approved -> "Approved"
Rejected -> "Rejected"
Reviewing -> "Reviewing"
readRequestStatus : String -> Maybe RequestStatus
readRequestStatus x = case x of
"Approved" -> Just Approved
"Rejected" -> Just Rejected
"Reviewing" -> Just Reviewing
_ -> Nothing
universeRequestStatus : List RequestStatus
universeRequestStatus = [Approved, Rejected, Reviewing]
encodeRequestStatus : RequestStatus -> Value
encodeRequestStatus = E.string << showRequestStatus
decodeRequestStatus : Decoder RequestStatus
decodeRequestStatus = elmStreetDecodeEnum readRequestStatus
Newtypes
Haskell
newtype Age = Age
{ unAge :: Int
} deriving (Generic)
deriving newtype (FromJSON, ToJSON)
deriving anyclass (Elm)
Elm
type alias Age =
{ age : Int
}
encodeAge : Age -> Value
encodeAge x = E.int x.age
decodeAge : Decoder Age
decodeAge = D.map Age D.int
Newtypes with phantom types
Haskell
newtype Id a = Id
{ unId :: Text
} deriving (Generic)
deriving newtype (FromJSON, ToJSON)
instance Elm (Id a) where
toElmDefinition _ = elmNewtype @Text "Id" "unId"
Elm
type alias Id =
{ unId : String
}
encodeId : Id -> Value
encodeId x = E.string x.unId
decodeId : Decoder Id
decodeId = D.map Id D.string
Sum types
Haskell
data Guest
= Regular Text Int
| Visitor Text
| Blocked
deriving (Generic)
deriving (Elm, ToJSON, FromJSON) via ElmStreet Guest
Elm
type Guest
= Regular String Int
| Visitor String
| Blocked
encodeGuest : Guest -> Value
encodeGuest x = E.object <| case x of
Regular x1 x2 -> [("tag", E.string "Regular"), ("contents", E.list identity [E.string x1, E.int x2])]
Visitor x1 -> [("tag", E.string "Visitor"), ("contents", E.string x1)]
Blocked -> [("tag", E.string "Blocked"), ("contents", E.list identity [])]
decodeGuest : Decoder Guest
decodeGuest =
let decide : String -> Decoder Guest
decide x = case x of
"Regular" -> D.field "contents" <| D.map2 Regular (D.index 0 D.string) (D.index 1 D.int)
"Visitor" -> D.field "contents" <| D.map Visitor D.string
"Blocked" -> D.succeed Blocked
c -> D.fail <| "Guest doesn't have such constructor: " ++ c
in D.andThen decide (D.field "tag" D.string)
*Note that all licence references and agreements mentioned in the elm-street README section above
are relevant to that project's source code only.