Servant Elm
Generate Elm functions to query your Servant API!
Elm type generation coutesy of krisajenkins/elm-export.
Installation
Servant Elm is available on Hackage.
Example
First, some language pragmas and imports.
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators #-}
import Elm (Spec (Spec), specsToDir, toElmDecoderSource,
toElmTypeSource)
import GHC.Generics (Generic)
import Servant.API ((:>), Capture, Get, JSON)
import Servant.Elm (ElmType, Proxy (Proxy), defElmImports,
generateElmForAPI)
We have some Haskell-defined types and our Servant API.
data Book = Book
{ name :: String
} deriving (Generic)
instance ElmType Book
type BooksApi = "books" :> Capture "bookId" Int :> Get '[JSON] Book
Now we can generate Elm functions to query the API:
spec :: Spec
spec = Spec ["Generated", "MyApi"]
(defElmImports
: toElmTypeSource (Proxy :: Proxy Book)
: toElmDecoderSource (Proxy :: Proxy Book)
: generateElmForAPI (Proxy :: Proxy BooksApi))
main :: IO ()
main = specsToDir [spec] "my-elm-dir"
Let's save this as example.hs
and run it:
$ stack runghc example.hs
Writing: my-elm-dir/Generated/MyApi.elm
$
Here's what was generated:
module Generated.MyApi exposing (..)
import Json.Decode exposing (..)
import Json.Decode.Pipeline exposing (..)
import Json.Encode
import Http
import String.Conversions as String
import Url
type alias Book =
{ name : String
}
decodeBook : Decoder Book
decodeBook =
decode Book
|> required "name" string
getBooksByBookId : Int -> Http.Request (Book)
getBooksByBookId capture_bookId =
Http.request
{ method =
"GET"
, headers =
[]
, url =
String.join "/"
[ ""
, "books"
, capture_bookId |> String.fromInt |> Url.percentEncode
]
, body =
Http.emptyBody
, expect =
Http.expectJson decodeBook
, timeout =
Nothing
, withCredentials =
False
}
See examples
for a complete usage example, or take a look at
mattjbray/servant-elm-example-app
for an example project using this library.
Development
$ git clone https://github.com/mattjbray/servant-elm.git
$ cd servant-elm
$ stack test
$ stack test --flag servant-elm:integration
To build all examples:
$ make examples
To run an example:
$ cd examples/e2e-tests
$ elm-reactor
# Open http://localhost:8000/elm/Main.elm