205 lines
7.1 KiB
Text
205 lines
7.1 KiB
Text
# Generating mock curl calls
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In this example we will generate curl requests with mock post data from a servant API.
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This may be useful for testing and development purposes.
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Especially post requests with a request body are tedious to send manually.
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Also, we will learn how to use the servant-foreign library to generate stuff from servant APIs.
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Language extensions and imports:
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``` haskell
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE DeriveGeneric #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE FlexibleInstances #-}
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{-# LANGUAGE GeneralizedNewtypeDeriving #-}
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{-# LANGUAGE MultiParamTypeClasses #-}
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{-# LANGUAGE OverloadedStrings #-}
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{-# LANGUAGE RankNTypes #-}
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{-# LANGUAGE RecordWildCards #-}
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{-# LANGUAGE ScopedTypeVariables #-}
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{-# LANGUAGE TypeOperators #-}
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import Control.Lens ((^.))
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import Data.Aeson
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import Data.Aeson.Text
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import Data.Proxy (Proxy (Proxy))
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import Data.Text (Text)
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import Data.Text.Encoding (decodeUtf8)
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import qualified Data.Text.IO as T.IO
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import qualified Data.Text.Lazy as LazyT
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import GHC.Generics
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import Servant ((:<|>), (:>), Get, JSON,
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Post, ReqBody)
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import Servant.Foreign (Foreign, GenerateList,
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HasForeign, HasForeignType, Req,
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Segment, SegmentType (Cap, Static),
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argName, listFromAPI, path,
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reqBody, reqMethod, reqUrl, typeFor,
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unPathSegment, unSegment,)
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import Test.QuickCheck.Arbitrary
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import Test.QuickCheck.Arbitrary.Generic
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import Test.QuickCheck.Gen (generate)
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import qualified Data.Text as T
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```
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Let's define our API:
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``` haskell
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type UserAPI = "users" :> Get '[JSON] [User]
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:<|> "new" :> "user" :> ReqBody '[JSON] User :> Post '[JSON] ()
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data User = User
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{ name :: String
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, age :: Int
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, email :: String
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} deriving (Eq, Show, Generic)
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instance Arbitrary User where
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arbitrary = genericArbitrary
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shrink = genericShrink
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instance ToJSON User
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instance FromJSON User
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```
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Notice the `Arbitrary User` instance which we will later need to create mock data.
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Also, the obligatory servant boilerplate:
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``` haskell
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api :: Proxy UserAPI
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api = Proxy
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```
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## servant-foreign and the HasForeignType Class
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Servant-foreign allows us to look into the API we designed.
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The entry point is `listFromAPI` which takes three types and returns a list of endpoints:
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``` haskell ignore
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listFromAPI :: (HasForeign lang ftype api, GenerateList ftype (Foreign ftype api)) => Proxy lang -> Proxy ftype -> Proxy api -> [Req ftype]
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```
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This looks a bit confusing...
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[Here](https://hackage.haskell.org/package/servant-foreign/docs/Servant-Foreign.html#t:HasForeign) is the documentation for the `HasForeign` typeclass.
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We will not go into details here, but this allows us to create a value of type `ftype` for any type `a` in our API.
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In our case we want to create a mock of every type `a`.
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We create a new datatype that holds our mocked value. Well, not the mocked value itself. To mock it we need IO (random). So the promise of a mocked value after some IO is performed:
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``` haskell
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data NoLang
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data Mocked = Mocked (IO Text)
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```
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Now, we create an instance of `HasForeignType` for `NoLang` and `Mocked` for every `a` that implements `ToJSON` and `Arbitrary`:
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``` haskell
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instance (ToJSON a, Arbitrary a) => HasForeignType NoLang Mocked a where
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typeFor _ _ _ =
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Mocked (genText (Proxy :: Proxy a))
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```
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What does `genText` do? It generates an arbitrary value of type `a` and encodes it as text. (And does some lazy to non-lazy text transformation we do not care about):
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``` haskell
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genText :: (ToJSON a, Arbitrary a) => Proxy a -> IO Text
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genText p =
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fmap (\v -> LazyT.toStrict $ encodeToLazyText v) (genArb p)
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genArb :: Arbitrary a => Proxy a -> IO a
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genArb _ =
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generate arbitrary
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```
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### Generating curl calls for every endpoint
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Everything is prepared now and we can start generating some curl calls.
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``` haskell
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generateCurl :: (GenerateList Mocked (Foreign Mocked api), HasForeign NoLang Mocked api)
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=> Proxy api
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-> Text
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-> IO Text
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generateCurl p host =
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fmap T.unlines body
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where
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body = mapM (generateEndpoint host)
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$ listFromAPI (Proxy :: Proxy NoLang) (Proxy :: Proxy Mocked) p
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```
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First, `listFromAPI` gives us a list of `Req Mocked`. Each `Req` describes one endpoint from the API type.
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We generate a curl call for each of them using the following helper.
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``` haskell
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generateEndpoint :: Text -> Req Mocked -> IO Text
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generateEndpoint host req =
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case maybeBody of
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Just body ->
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body >>= \b -> return $ T.intercalate " " [ "curl", "-X", method, "-d", "'" <> b <> "'"
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, "-H 'Content-Type: application/json'", host <> "/" <> url ]
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Nothing ->
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return $ T.intercalate " " [ "curl", "-X", method, host <> "/" <> url ]
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where
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method = decodeUtf8 $ req ^. reqMethod
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url = T.intercalate "/" $ map segment (req ^. reqUrl . path)
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maybeBody = fmap (\(Mocked io) -> io) (req ^. reqBody)
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```
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`servant-foreign` offers a multitude of lenses to be used with `Req`-values.
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`reqMethod` gives us a straigthforward `Network.HTTP.Types.Method`, `reqUrl` the url part and so on.
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Just take a look at [the docs](https://hackage.haskell.org/package/servant-foreign/docs/Servant-Foreign.html).
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But how do we get our mocked json string? This seems to be a bit to short to be true:
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``` haskell ignore
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maybeBody = fmap (\(Mocked io) -> io) (req ^. reqBody)
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```
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But it is that simple!
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The docs say `reqBody` gives us a `Maybe f`. What is `f`, you ask? As defined in `generateCurl`, `f` is `Mocked` and contains a `IO Text`. How is this `Mocked` value created? The `HasForeignType::typeFor` does it!
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Of course only if the endpoint has a request body.
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Some (incomplete) code for url segments:
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``` haskell
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segment :: Segment Mocked -> Text
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segment seg =
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case unSegment seg of
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Static p ->
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unPathSegment p
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Cap arg ->
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-- Left as exercise for the reader: Mock args in the url
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unPathSegment $ arg ^. argName
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```
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And now, lets hook it all up in our main function:
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``` haskell
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main :: IO ()
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main =
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generateCurl api "localhost:8081" >>= T.IO.putStrLn
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```
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Done:
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``` curl
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curl -X GET localhost:8081/users
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curl -X POST -d '{"email":"wV_b:z!(3DM V","age":10,"name":"=|W"}' -H 'Content-Type: application/json' localhost:8081/new/user
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```
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This is of course no complete curl call mock generator, many things including path arguments are missing.
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But it correctly generates mock calls for simple POST requests.
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Also, we now know how to use `HasForeignType` and `listFromAPI` to generate anything we want.
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