Ethereum JSON-RPC query example on Haskell

Main.hs

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Numeric (showHex)
import Data.Text (pack)
import Control.Monad.IO.Class
import Data.Aeson
import Network.HTTP.Req

data Request = BlockNumberReq | BlockByNumberReq Int Bool

{-| Decode

>>> decode "{\"jsonrpc\":\"2.0\",\"result\":\"0xb27398\",\"id\":1}" :: Maybe Response
-}

data Response = Response {id :: Int, jsonrpc :: String, result :: String} deriving (Show)

instance FromJSON Response where
    parseJSON (Object o) =
        Response <$> 
        o .: "id" <*> 
        o .: "jsonrpc" <*> 
        o .: "result"

buildRequest :: Request -> Int -> ReqBodyJson Value
buildRequest BlockNumberReq id = buildRPC "eth_blockNumber" [] id
buildRequest (BlockByNumberReq num full) id = buildRPC "eth_getBlockByNumber" [String $ pack $ "0x" ++ showHex num "", Bool full] id

buildRPC :: String -> [Value] -> Int -> ReqBodyJson Value
buildRPC method params id =
    ReqBodyJson $ object ["method" .= method, "params" .= params, "id" .= id, "jsonrpc" .= String "2.0"]

main :: IO ()
-- You can either make your monad an instance of 'MonadHttp', or use
-- 'runReq' in any IO-enabled monad without defining new instances.
main = runReq defaultHttpConfig $ do
  -- One function—full power and flexibility, automatic retrying on timeouts
  -- and such, automatic connection sharing.
  r <-
    req
      POST -- method
      (http "localhost") -- safe by construction URL
      (buildRequest BlockNumberReq 1) -- use built-in options or add your own
      jsonResponse -- specify how to interpret response
      (port 8545) -- query params, headers, explicit port number, etc.
  liftIO $ print (responseBody r :: Response)