A web API as a type

The source for this tutorial section is a literate haskell file, so first we need to have some language extensions and imports:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}

module ApiType where

import Data.Text
import Data.Time (UTCTime)
import Servant.API

Consider the following informal specification of an API:

The endpoint at /users expects a GET request with query string parameter sortby whose value can be one of age or name and returns a list/array of JSON objects describing users, with fields age, name, email, registration_date”.

You should be able to formalize that. And then use the formalized version to get you much of the way towards writing a web app. And all the way towards getting some client libraries, and documentation, and more.

How would we describe it with servant? An endpoint description is a good old Haskell type:

type UserAPI = "users" :> QueryParam "sortby" SortBy :> Get '[JSON] [User]

data SortBy = Age | Name

data User = User {
  name :: String,
  age :: Int,
  email :: String,
  registration_date :: UTCTime

Let’s break that down:

  • "users" says that our endpoint will be accessible under /users;
  • QueryParam "sortby" SortBy, where SortBy is defined by data SortBy = Age | Name, says that the endpoint has a query string parameter named sortby whose value will be extracted as a value of type SortBy.
  • Get '[JSON] [User] says that the endpoint will be accessible through HTTP GET requests, returning a list of users encoded as JSON. You will see later how you can make use of this to make your data available under different formats, the choice being made depending on the Accept header specified in the client’s request.
  • The :> operator that separates the various “combinators” just lets you sequence static path fragments, URL captures and other combinators. The ordering only matters for static path fragments and URL captures. "users" :> "list-all" :> Get '[JSON] [User], equivalent to /users/list-all, is obviously not the same as "list-all" :> "users" :> Get '[JSON] [User], which is equivalent to /list-all/users. This means that sometimes :> is somehow equivalent to /, but sometimes it just lets you chain another combinator.

Tip: If your endpoint responds to / (the root path), just omit any combinators that introduce path segments. E.g. the following api has only one endpoint on /:

type RootEndpoint =
  Get '[JSON] User

We can also describe APIs with multiple endpoints by using the :<|> combinators. Here’s an example:

type UserAPI2 = "users" :> "list-all" :> Get '[JSON] [User]
           :<|> "list-all" :> "users" :> Get '[JSON] [User]

servant provides a fair amount of combinators out-of-the-box, but you can always write your own when you need it. Here’s a quick overview of the most often needed combinators that servant comes with.


Static strings

As you’ve already seen, you can use type-level strings (enabled with the DataKinds language extension) for static path fragments. Chaining them amounts to /-separating them in a URL.

type UserAPI3 = "users" :> "list-all" :> "now" :> Get '[JSON] [User]
              -- describes an endpoint reachable at:
              -- /users/list-all/now

Delete, Get, Patch, Post and Put

The Get combinator is defined in terms of the more general Verb:

data Verb method (statusCode :: Nat) (contentType :: [*]) a
type Get = Verb 'GET 200

There are other predefined type synonyms for other common HTTP methods, such as e.g.:

type Delete = Verb 'DELETE 200
type Patch  = Verb 'PATCH 200
type Post   = Verb 'POST 200
type Put    = Verb 'PUT 200

There are also variants that do not return a 200 status code, such as for example:

type PostCreated  = Verb 'POST 201
type PostAccepted = Verb 'POST 202

An endpoint always ends with a variant of the Verb combinator (unless you write your own combinators). Examples:

type UserAPI4 = "users" :> Get '[JSON] [User]
           :<|> "admins" :> Get '[JSON] [User]


URL captures are segments of the path of a URL that are variable and whose actual value is captured and passed to the request handlers. In many web frameworks, you’ll see it written as in /users/:userid, with that leading : denoting that userid is just some kind of variable name or placeholder. For instance, if userid is supposed to range over all integers greater or equal to 1, our endpoint will match requests made to /users/1, /users/143 and so on.

The Capture combinator in servant takes a (type-level) string representing the “name of the variable” and a type, which indicates the type we want to decode the “captured value” to.

data Capture (s :: Symbol) a
-- s :: Symbol just says that 's' must be a type-level string.

In some web frameworks, you use regexes for captures. We use a FromHttpApiData class, which the captured value must be an instance of.


type UserAPI5 = "user" :> Capture "userid" Integer :> Get '[JSON] User
                -- equivalent to 'GET /user/:userid'
                -- except that we explicitly say that "userid"
                -- must be an integer

           :<|> "user" :> Capture "userid" Integer :> DeleteNoContent '[JSON] NoContent
                -- equivalent to 'DELETE /user/:userid'

In the second case, DeleteNoContent specifies a 204 response code, JSON specifies the content types on which the handler will match, and NoContent says that the response will always be empty.

QueryParam, QueryParams, QueryFlag

QueryParam, QueryParams and QueryFlag are about parameters in the query string, i.e., those parameters that come after the question mark (?) in URLs, like sortby in /users?sortby=age, whose value is set to age. QueryParams lets you specify that the query parameter is actually a list of values, which can be specified using ?param=value1&param=value2. This represents a list of values composed of value1 and value2. QueryFlag lets you specify a boolean-like query parameter where a client isn’t forced to specify a value. The absence or presence of the parameter’s name in the query string determines whether the parameter is considered to have the value True or False. For instance, /users?active would list only active users whereas /users would list them all.

Here are the corresponding data type declarations:

data QueryParam (sym :: Symbol) a
data QueryParams (sym :: Symbol) a
data QueryFlag (sym :: Symbol)


type UserAPI6 = "users" :> QueryParam "sortby" SortBy :> Get '[JSON] [User]
                -- equivalent to 'GET /users?sortby={age, name}'

Again, your handlers don’t have to deserialize these things (into, for example, a SortBy). servant takes care of it.


Each HTTP request can carry some additional data that the server can use in its body, and this data can be encoded in any format – as long as the server understands it. This can be used for example for an endpoint for creating new users: instead of passing each field of the user as a separate query string parameter or something dirty like that, we can group all the data into a JSON object. This has the advantage of supporting nested objects.

servant‘s ReqBody combinator takes a list of content types in which the data encoded in the request body can be represented and the type of that data. And, as you might have guessed, you don’t have to check the content type header, and do the deserialization yourself. We do it for you. And return Bad Request or Unsupported Content Type as appropriate.

Here’s the data type declaration for it:

data ReqBody (contentTypes :: [*]) a


type UserAPI7 = "users" :> ReqBody '[JSON] User :> Post '[JSON] User
                -- - equivalent to 'POST /users' with a JSON object
                --   describing a User in the request body
                -- - returns a User encoded in JSON

           :<|> "users" :> Capture "userid" Integer
                        :> ReqBody '[JSON] User
                        :> Put '[JSON] User
                -- - equivalent to 'PUT /users/:userid' with a JSON
                --   object describing a User in the request body
                -- - returns a User encoded in JSON

Request Headers

Request headers are used for various purposes, from caching to carrying auth-related data. They consist of a header name and an associated value. An example would be Accept: application/json.

The Header combinator in servant takes a type-level string for the header name and the type to which we want to decode the header’s value (from some textual representation), as illustrated below:

data Header (sym :: Symbol) a

Here’s an example where we declare that an endpoint makes use of the User-Agent header which specifies the name of the software/library used by the client to send the request.

type UserAPI8 = "users" :> Header "User-Agent" Text :> Get '[JSON] [User]

Content types

So far, whenever we have used a combinator that carries a list of content types, we’ve always specified '[JSON]. However, servant lets you use several content types, and also lets you define your own content types.

Four content types are provided out-of-the-box by the core servant package: JSON, PlainText, FormUrlEncoded and OctetStream. If for some obscure reason you wanted one of your endpoints to make your user data available under those 4 formats, you would write the API type as below:

type UserAPI9 = "users" :> Get '[JSON, PlainText, FormUrlEncoded, OctetStream] [User]

(There are other packages that provide other content types. For example servant-lucid and servant-blaze allow to generate html pages (using lucid and blaze-html) and both come with a content type for html.)

We will further explain how these content types and your data types can play together in the section about serving an API.

Response Headers

Just like an HTTP request, the response generated by a webserver can carry headers too. servant provides a Headers combinator that carries a list of Header types and can be used by simply wrapping the “return type” of an endpoint with it.

data Headers (ls :: [*]) a

If you want to describe an endpoint that returns a “User-Count” header in each response, you could write it as below:

type UserAPI10 = "users" :> Get '[JSON] (Headers '[Header "User-Count" Integer] [User])

Basic Authentication

Once you’ve established the basic routes and semantics of your API, it’s time to consider protecting parts of it. Authentication and authorization are broad and nuanced topics; as servant began to explore this space we started small with one of HTTP’s earliest authentication schemes: Basic Authentication.

When protecting endpoints with basic authentication, we need to specify two items:

  1. The realm of authentication as per the Basic Authentication spec.
  2. The datatype returned by the server after authentication is verified. This is usually a User or Customer type datatype.

With those two items in mind, servant provides the following combinator:

data BasicAuth (realm :: Symbol) (userData :: *)

Which is used like so:

type ProtectedAPI11
     = UserAPI                              -- this is public
 :<|> BasicAuth "my-realm" User :> UserAPI2 -- this is protected by auth

Empty APIs

Sometimes it is useful to be able to generalise an API over the type of some part of it:

type UserAPI12 innerAPI
     = UserAPI             -- this is the fixed bit of the API
 :<|> "inner" :> innerAPI  -- this lets us put various other APIs under /inner

If there is a case where you do not have anything extra to serve, you can use the EmptyAPI combinator to indicate this:

type UserAPI12Alone = UserAPI12 EmptyAPI

This also works well as a placeholder for unfinished parts of an API while it is under development, for when you know that there should be something there but you don’t yet know what. Think of it as similar to the unit type ().

Interoperability with wai: Raw

Finally, we also include a combinator named Raw that provides an escape hatch to the underlying low-level web library wai. It can be used when you want to plug a wai Application into your webservice:

type UserAPI13 = "users" :> Get '[JSON] [User]
                 -- a /users endpoint

            :<|> Raw
                 -- requests to anything else than /users
                 -- go here, where the server will try to
                 -- find a file with the right name
                 -- at the right path

One example for this is if you want to serve a directory of static files along with the rest of your API. But you can plug in everything that is an Application, e.g. a whole web application written in any of the web frameworks that support wai.