I would like to use servant, in particular implement a literate haskell file. I cannot figure out how to use the literate haskell file. I've been searching for documentation but nothing helpful has come up.
So far I have named the file correctly with the extension .lhs and I have executed runhaskell filename.lhs. I am receiving the following error:
servantfinaltest.lhs line 150: unlit: No definitions in file (perhaps you forgot the '>'s?)
`unlit' failed in phase `Literate pre-processor'. (Exit code: 1)
This is my .lhs file below:
# Serving an API
Enough chit-chat about type-level combinators and representing an API as a
type. Can we have a webservice already?
## A first example
Equipped with some basic knowledge about the way we represent APIs, let's now
write our first webservice.
The source for this tutorial section is a literate haskell file, so first we
need to have some language extensions and imports:
``` haskell
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
module Server where
import Prelude ()
import Prelude.Compat
import Control.Monad.Except
import Control.Monad.Reader
import Data.Aeson.Compat
import Data.Aeson.Types
import Data.Attoparsec.ByteString
import Data.ByteString (ByteString)
import Data.List
import Data.Maybe
import Data.String.Conversions
import Data.Time.Calendar
import GHC.Generics
import Lucid
import Network.HTTP.Media ((//), (/:))
import Network.Wai
import Network.Wai.Handler.Warp
import Servant
import System.Directory
import Text.Blaze
import Text.Blaze.Html.Renderer.Utf8
import qualified Data.Aeson.Parser
import qualified Text.Blaze.Html
```
**Important**: the `Servant` module comes from the **servant-server** package,
the one that lets us run webservers that implement a particular API type. It
reexports all the types from the **servant** package that let you declare API
types as well as everything you need to turn your request handlers into a
fully-fledged webserver. This means that in your applications, you can just add
**servant-server** as a dependency, import `Servant` and not worry about anything
else.
We will write a server that will serve the following API.
``` haskell
type UserAPI1 = "users" :> Get '[JSON] [User]
```
Here's what we would like to see when making a GET request to `/users`.
``` javascript
[ {"name": "Isaac Newton", "age": 372, "email": "[email protected]", "registration_date": "1683-03-01"}
, {"name": "Albert Einstein", "age": 136, "email": "[email protected]", "registration_date": "1905-12-01"}
]
```
Now let's define our `User` data type and write some instances for it.
``` haskell
data User = User
{ name :: String
, age :: Int
, email :: String
, registration_date :: Day
} deriving (Eq, Show, Generic)
instance ToJSON User
```
Nothing funny going on here. But we now can define our list of two users.
``` haskell
users1 :: [User]
users1 =
[ User "Isaac Newton" 372 "[email protected]" (fromGregorian 1683 3 1)
, User "Albert Einstein" 136 "[email protected]" (fromGregorian 1905 12 1)
]
```
Let's also write our API type.
``` haskell ignore
type UserAPI1 = "users" :> Get '[JSON] [User]
```
We can now take care of writing the actual webservice that will handle requests
to such an API. This one will be very simple, being reduced to just a single
endpoint. The type of the web application is determined by the API type,
through a *type family* named `Server`. (Type families are just functions that
take types as input and return types.) The `Server` type family will compute
the right type that a bunch of request handlers should have just from the
corresponding API type.
The first thing to know about the `Server` type family is that behind the
scenes it will drive the routing, letting you focus only on the business
logic. The second thing to know is that for each endpoint, your handlers will
by default run in the `Handler` monad. This is overridable very
easily, as explained near the end of this guide. Third thing, the type of the
value returned in that monad must be the same as the second argument of the
HTTP method combinator used for the corresponding endpoint. In our case, it
means we must provide a handler of type `Handler [User]`. Well,
we have a monad, let's just `return` our list:
``` haskell
server1 :: Server UserAPI1
server1 = return users1
```
That's it. Now we can turn `server` into an actual webserver using
[wai](http://hackage.haskell.org/package/wai) and
[warp](http://hackage.haskell.org/package/warp):
``` haskell
userAPI :: Proxy UserAPI1
userAPI = Proxy
-- 'serve' comes from servant and hands you a WAI Application,
-- which you can think of as an "abstract" web application,
-- not yet a webserver.
app1 :: Application
app1 = serve userAPI server1
```
The `userAPI` bit is, alas, boilerplate (we need it to guide type inference).
But that's about as much boilerplate as you get.
And we're done! Let's run our webservice on the port 8081.
``` haskell
main :: IO ()
main = run 8081 app1
```
First of all - if you have not written any haskell code - starting with servant and is I'd say quite ambitious - as it leverages several high level concepts/mechanisms provided by several language extensions like
TypeFamiliesandDataKinds...What you are writing is no literate haskell file - at least it violates the syntax as described here
I recommend either sticking to normal haskell files or reading the document I linked to first.
Here is a translation of your file into valid literate haskell: