servant-reason: Derive Reason types to interact with a Haskell backend

[ bsd3, library, web ] [ Propose Tags ]

Please see the README on GitHub at

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Dependencies base (>=4.7 && <5), lens, reason-export (>=, servant, servant-foreign, text, wl-pprint-text [details]
License BSD-3-Clause
Copyright 2019 Andrei Barbu
Author Andrei Barbu
Category Web
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Source repo head: git clone
Uploaded by AndreiBarbu at 2019-06-01T07:57:37Z
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Last success reported on 2019-06-01 [all 1 reports]


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Servant Reason

Build Status <img src="" alt="reason" height="20"/> <img src="" alt="reason" height="20"/>

Automatically derive bindings for Servant APIs in Reason. Originally build by converting servant-elm to Reason. Types are generated using reason-export.

More docs on Hackage.

A full example of how to integrate Servant and Reason is available at servant-reason-example.


Run the tests if you want a concrete example. They will build a _test-cache directory that contains a sample Reason repository for a servant API. test also contain example output files for different kinds of bindings.

Usage is simple and automatic. If you have an API like the one below

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators     #-}

import           Reason          (Spec (Spec), specsToDir, toReasonDecoderSource,
import           GHC.Generics (Generic)
import           Servant.API  ((:>), Capture, Get, JSON)
import           Servant.Reason  (ReasonType, Proxy (Proxy), defReasonImports,

data Book = Book
    { name :: String
    } deriving (Generic)

instance ReasonType Book

type BooksApi = "books" :> Capture "bookId" Int :> Get '[JSON] Book

You can expose your API to reason with:

module Main where

import Data.Proxy
import Reason
import           Data.Text hiding (intercalate, map)
import Db

main :: IO ()
main = do
  let code = defReasonImports
             : toReasonTypeSource    (Proxy :: Proxy Book)
             : toReasonDecoderSource (Proxy :: Proxy Book)
             : generateReasonForAPI  (Proxy :: Proxy BooksApi))
  writeFile "" $ intercalate "\n\n" $ map unpack code

That's it. You can now include in a Reason project.

Reason setup

The generated code needs access to two Reason libraries @glennsl/bs-json and bs-fetch. Get the latest install instructions from the upstream repos, but at the time of writing these were:

npm install --save @glennsl/bs-json
npm install --save bs-fetch

Then add @glennsl/bs-json and bs-fetch to bs-dependencies in your bsconfig.json:

  "bs-dependencies": ["@glennsl/bs-json", "bs-fetch"]