# twirp-haskell

Very, very alpha implementation of Twirp service generation for Haskell.

This project provides a number of things:

1. A protoc plugin for generating a Haskell Twirp service (based on Servant) for Services defined in a proto file.
2. A haskell library, `twirp` for quickly serving up the generated application.

It requires the use of [proto-lens](https://github.com/google/proto-lens) to generate haskell datatypes from proto messages.

An example, end-to-end application is included in `app`.

## Requirements

1. Install `protoc` (e.g., `brew install protoc`)
2. Install the required protoc plugins:
   - `cabal install proto-lens-protoc`
   - `go get github.com/tclem/twirp-haskell/protoc-gen-twirp_haskell`
   - `go get github.com/tclem/proto-lens-jsonpb/protoc-gen-jsonpb_haskell`

## Usage

Use the protoc plugin to generate a twirp service and associated protobuf types from a proto file.

```
protoc -I=. --proto_path=./proto \
  --plugin=protoc-gen-haskell=`which proto-lens-protoc` --haskell_out=./app \
  --jsonpb_haskell_out=./app \
  --plugin=protoc-gen-twirp_haskell=./script/run-twirp_haskell --twirp_haskell_out=./app/Twirp/Example/Haberdasher \
  haberdasher.proto
```

The result is a couple of files that describe your service. First, here are the types that define a Servant API:


``` haskell
-- Code generated by protoc-gen-twirp_haskell 0.1.0, DO NOT EDIT.
{-# LANGUAGE TypeOperators #-}
module Twirp.Example.Haberdasher.Haberdasher where

import Servant
import Twirp

import Proto.Haberdasher

--  This is an example set of twirp services.

type HaberdasherAPI
  =    "twirp" :> "twirp.example.haberdasher.Haberdasher" :> HaberdasherService
  :<|> "twirp" :> "twirp.example.haberdasher.Health" :> HealthService

--  Haberdasher service makes hats for clients.
type HaberdasherService
  --  MakeHat produces a hat of mysterious, randomly-selected color!
  =    "MakeHat" :> ReqBody [Protobuf, JSON] Size :> Post '[Protobuf, JSON] Hat
  --  Get paid
  :<|> "GetBill" :> ReqBody [Protobuf, JSON] Hat :> Post '[Protobuf, JSON] Bill

--  Health check service
type HealthService
  =    "Check" :> ReqBody [Protobuf, JSON] Ping :> Post '[Protobuf, JSON] Pong
```

The datatypes are defined in [`Proto.Haberdasher`](app/Proto/Haberdasher.hs).

Plugging this into an existing warp/wai server is straightforward. See [`app/Main.hs`](app/Main.hs) for the full details:

``` haskell
type RequestID = String

type API
  = Header "X-Request-Id" RequestID
  :> HaberdasherAPI

main :: IO ()
main = run 8003 app

app :: Application
app = twirpErrorResponses apiApp

apiApp :: Application
apiApp = serve (Proxy :: Proxy API) server

server :: Server API
server _requestID = (makeHat :<|> getBill) :<|> checkHealth
```