Servant.OpenApi

This module provides means to generate and manipulate OpenApi specification for servant APIs.

OpenApi is a project used to describe and document RESTful APIs.

The OpenApi specification defines a set of files required to describe such an API. These files can then be used by the OpenApi-UI project to display the API and OpenApi-Codegen to generate clients in various languages. Additional utilities can also take advantage of the resulting files, such as testing tools.

For more information see OpenApi documentation.

How to use this library

This section explains how to use this library to generate OpenApi specification, modify it and run automatic tests for a servant API.

For the purposes of this section we will use this servant API:

>>> data User = User { name :: String, age :: Int } deriving (Show, Generic, Typeable)
>>> newtype UserId = UserId Integer deriving (Show, Generic, Typeable, ToJSON)
>>> instance ToJSON User
>>> instance ToSchema User
>>> instance ToSchema UserId
>>> instance ToParamSchema UserId
>>> type GetUsers = Get '[JSON] [User]
>>> type GetUser  = Capture "user_id" UserId :> Get '[JSON] User
>>> type PostUser = ReqBody '[JSON] User :> Post '[JSON] UserId
>>> type UserAPI  = GetUsers :<|> GetUser :<|> PostUser

Here we define a user API with three endpoints. GetUsers endpoint returns a list of all users. GetUser returns a user given his/her ID. PostUser creates a new user and returns his/her ID.

Generate OpenApi

In order to generate OpenApi specification for a servant API, just use toOpenApi:

>>> BSL8.putStrLn $ encodePretty $ toOpenApi (Proxy :: Proxy UserAPI)
{
    "components": {
        "schemas": {
            "User": {
                "properties": {
                    "age": {
                        "maximum": 9223372036854775807,
                        "minimum": -9223372036854775808,
                        "type": "integer"
                    },
                    "name": {
                        "type": "string"
                    }
                },
                "required": [
                    "name",
                    "age"
                ],
                "type": "object"
            },
            "UserId": {
                "type": "integer"
            }
        }
    },
    "info": {
        "title": "",
        "version": ""
    },
    "openapi": "3.0.0",
    "paths": {
        "/": {
            "get": {
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "items": {
                                        "$ref": "#/components/schemas/User"
                                    },
                                    "type": "array"
                                }
                            }
                        },
                        "description": ""
                    }
                }
            },
            "post": {
                "requestBody": {
                    "content": {
                        "application/json;charset=utf-8": {
                            "schema": {
                                "$ref": "#/components/schemas/User"
                            }
                        }
                    }
                },
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/UserId"
                                }
                            }
                        },
                        "description": ""
                    },
                    "400": {
                        "description": "Invalid `body`"
                    }
                }
            }
        },
        "/{user_id}": {
            "get": {
                "parameters": [
                    {
                        "in": "path",
                        "name": "user_id",
                        "required": true,
                        "schema": {
                            "type": "integer"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/User"
                                }
                            }
                        },
                        "description": ""
                    },
                    "404": {
                        "description": "`user_id` not found"
                    }
                }
            }
        }
    }
}

By default toOpenApi will generate specification for all API routes, parameters, headers, responses and data schemas.

For some parameters it will also add 400 and/or 404 responses with a description mentioning parameter name.

Data schemas come from ToParamSchema and ToSchema classes.

Annotate

While initially generated OpenApi looks good, it lacks some information it can't get from a servant API.

We can add this information using field lenses from Data.OpenApi:

>>> :{
BSL8.putStrLn $ encodePretty $ toOpenApi (Proxy :: Proxy UserAPI)
  & info.title        .~ "User API"
  & info.version      .~ "1.0"
  & info.description  ?~ "This is an API for the Users service"
  & info.license      ?~ "MIT"
  & servers           .~ ["https://example.com"]
:}
{
    "components": {
        "schemas": {
            "User": {
                "properties": {
                    "age": {
                        "maximum": 9223372036854775807,
                        "minimum": -9223372036854775808,
                        "type": "integer"
                    },
                    "name": {
                        "type": "string"
                    }
                },
                "required": [
                    "name",
                    "age"
                ],
                "type": "object"
            },
            "UserId": {
                "type": "integer"
            }
        }
    },
    "info": {
        "description": "This is an API for the Users service",
        "license": {
            "name": "MIT"
        },
        "title": "User API",
        "version": "1.0"
    },
    "openapi": "3.0.0",
    "paths": {
        "/": {
            "get": {
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "items": {
                                        "$ref": "#/components/schemas/User"
                                    },
                                    "type": "array"
                                }
                            }
                        },
                        "description": ""
                    }
                }
            },
            "post": {
                "requestBody": {
                    "content": {
                        "application/json;charset=utf-8": {
                            "schema": {
                                "$ref": "#/components/schemas/User"
                            }
                        }
                    }
                },
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/UserId"
                                }
                            }
                        },
                        "description": ""
                    },
                    "400": {
                        "description": "Invalid `body`"
                    }
                }
            }
        },
        "/{user_id}": {
            "get": {
                "parameters": [
                    {
                        "in": "path",
                        "name": "user_id",
                        "required": true,
                        "schema": {
                            "type": "integer"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/User"
                                }
                            }
                        },
                        "description": ""
                    },
                    "404": {
                        "description": "`user_id` not found"
                    }
                }
            }
        }
    },
    "servers": [
        {
            "url": "https://example.com"
        }
    ]
}

It is also useful to annotate or modify certain endpoints. subOperations provides a convenient way to zoom into a part of an API.

subOperations sub api traverses all operations of the api which are also present in sub. Furthermore, sub is required to be an exact sub API of @api. Otherwise it will not typecheck.

Data.OpenApi.Operation provides some useful helpers that can be used with subOperations. One example is applying tags to certain endpoints:

>>> let getOps  = subOperations (Proxy :: Proxy (GetUsers :<|> GetUser)) (Proxy :: Proxy UserAPI)
>>> let postOps = subOperations (Proxy :: Proxy PostUser) (Proxy :: Proxy UserAPI)
>>> :{
BSL8.putStrLn $ encodePretty $ toOpenApi (Proxy :: Proxy UserAPI)
  & applyTagsFor getOps  ["get"  & description ?~ "GET operations"]
  & applyTagsFor postOps ["post" & description ?~ "POST operations"]
:}
{
    "components": {
        "schemas": {
            "User": {
                "properties": {
                    "age": {
                        "maximum": 9223372036854775807,
                        "minimum": -9223372036854775808,
                        "type": "integer"
                    },
                    "name": {
                        "type": "string"
                    }
                },
                "required": [
                    "name",
                    "age"
                ],
                "type": "object"
            },
            "UserId": {
                "type": "integer"
            }
        }
    },
    "info": {
        "title": "",
        "version": ""
    },
    "openapi": "3.0.0",
    "paths": {
        "/": {
            "get": {
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "items": {
                                        "$ref": "#/components/schemas/User"
                                    },
                                    "type": "array"
                                }
                            }
                        },
                        "description": ""
                    }
                },
                "tags": [
                    "get"
                ]
            },
            "post": {
                "requestBody": {
                    "content": {
                        "application/json;charset=utf-8": {
                            "schema": {
                                "$ref": "#/components/schemas/User"
                            }
                        }
                    }
                },
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/UserId"
                                }
                            }
                        },
                        "description": ""
                    },
                    "400": {
                        "description": "Invalid `body`"
                    }
                },
                "tags": [
                    "post"
                ]
            }
        },
        "/{user_id}": {
            "get": {
                "parameters": [
                    {
                        "in": "path",
                        "name": "user_id",
                        "required": true,
                        "schema": {
                            "type": "integer"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "content": {
                            "application/json;charset=utf-8": {
                                "schema": {
                                    "$ref": "#/components/schemas/User"
                                }
                            }
                        },
                        "description": ""
                    },
                    "404": {
                        "description": "`user_id` not found"
                    }
                },
                "tags": [
                    "get"
                ]
            }
        }
    },
    "tags": [
        {
            "description": "GET operations",
            "name": "get"
        },
        {
            "description": "POST operations",
            "name": "post"
        }
    ]
}

This applies "get" tag to the GET endpoints and "post" tag to the POST endpoint of the User API.

Test

Automatic generation of data schemas uses ToSchema instances for the types used in a servant API. But to encode/decode actual data servant uses different classes. For instance in UserAPI User is always encoded/decoded using ToJSON and FromJSON instances.

To be sure your Haskell server/client handles data properly you need to check that ToJSON instance always generates values that satisfy schema produced by ToSchema instance.

With validateEveryToJSON it is possible to test all those instances automatically, without having to write down every type:

>>> instance Arbitrary User where arbitrary = User <$> arbitrary <*> arbitrary
>>> instance Arbitrary UserId where arbitrary = UserId <$> arbitrary
>>> hspec $ validateEveryToJSON (Proxy :: Proxy UserAPI)

[User]...
...
User...
...
UserId...
...
Finished in ... seconds
...3 examples, 0 failures...

Although servant is great, chances are that your API clients don't use Haskell. In many cases swagger.json serves as a specification, not a Haskell type.

In this cases it is a good idea to store generated and annotated OpenApi in a swagger.json file under a version control system (such as Git, Subversion, Mercurial, etc.).

It is also recommended to version API based on changes to the swagger.json rather than changes to the Haskell API.

See TodoSpec.hs for an example of a complete test suite for a swagger specification.

Serve

If you're implementing a server for an API, you might also want to serve its OpenApi specification.

See Todo.hs for an example of a server.

HasOpenApi class

class HasOpenApi api where

Generate a OpenApi specification for a servant API.

To generate OpenApi specification, your data types need ToParamSchema and/or ToSchema instances.

ToParamSchema is used for Capture, QueryParam and Header. ToSchema is used for ReqBody and response data types.

You can easily derive those instances via Generic. For more information, refer to openapi3 documentation.

Example:

newtype Username = Username String deriving (Generic, ToText)

instance ToParamSchema Username

data User = User
  { username :: Username
  , fullname :: String
  } deriving (Generic)

instance ToJSON User
instance ToSchema User

type MyAPI = QueryParam "username" Username :> Get '[JSON] User

myOpenApi :: OpenApi
myOpenApi = toOpenApi (Proxy :: Proxy MyAPI)

Methods

toOpenApi :: Proxy api -> OpenApi

Generate a OpenApi specification for a servant API.

Instances

HasOpenApi EmptyAPI

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy EmptyAPI -> OpenApi

HasOpenApi Raw

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy Raw -> OpenApi

HasOpenApi (ToServantApi sub) => HasOpenApi (NamedRoutes sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (NamedRoutes sub) -> OpenApi

(HasOpenApi a, HasOpenApi b) => HasOpenApi (a :<|> b :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (a :<|> b) -> OpenApi

OpenApiMethod method => HasOpenApi (NoContentVerb method :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (NoContentVerb method) -> OpenApi

HasOpenApi sub => HasOpenApi (Vault :> sub :: Type)

Vault combinator does not change our specification at all.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Vault :> sub) -> OpenApi

(KnownSymbol sym, HasOpenApi sub) => HasOpenApi (sym :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (sym :> sub) -> OpenApi

HasOpenApi sub => HasOpenApi (HttpVersion :> sub :: Type)

HttpVersion combinator does not change our specification at all.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (HttpVersion :> sub) -> OpenApi

(KnownSymbol sym, ToParamSchema a, HasOpenApi sub, KnownSymbol (FoldDescription mods)) => HasOpenApi (Capture' mods sym a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Capture' mods sym a :> sub) -> OpenApi

(KnownSymbol sym, ToParamSchema a, HasOpenApi sub) => HasOpenApi (CaptureAll sym a :> sub :: Type)

OpenApi Spec doesn't have a notion of CaptureAll, this instance is the best effort.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (CaptureAll sym a :> sub) -> OpenApi

(KnownSymbol desc, HasOpenApi api) => HasOpenApi (Description desc :> api :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Description desc :> api) -> OpenApi

(KnownSymbol desc, HasOpenApi api) => HasOpenApi (Summary desc :> api :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Summary desc :> api) -> OpenApi

HasOpenApi sub => HasOpenApi (Fragment a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Fragment a :> sub) -> OpenApi

(KnownSymbol sym, ToParamSchema a, HasOpenApi sub, SBoolI (FoldRequired mods), KnownSymbol (FoldDescription mods)) => HasOpenApi (Header' mods sym a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Header' mods sym a :> sub) -> OpenApi

HasOpenApi sub => HasOpenApi (IsSecure :> sub :: Type)

IsSecure combinator does not change our specification at all.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (IsSecure :> sub) -> OpenApi

(KnownSymbol sym, HasOpenApi sub) => HasOpenApi (QueryFlag sym :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (QueryFlag sym :> sub) -> OpenApi

(KnownSymbol sym, ToParamSchema a, HasOpenApi sub, SBoolI (FoldRequired mods), KnownSymbol (FoldDescription mods)) => HasOpenApi (QueryParam' mods sym a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (QueryParam' mods sym a :> sub) -> OpenApi

(KnownSymbol sym, ToParamSchema a, HasOpenApi sub) => HasOpenApi (QueryParams sym a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (QueryParams sym a :> sub) -> OpenApi

HasOpenApi sub => HasOpenApi (RemoteHost :> sub :: Type)

RemoteHost combinator does not change our specification at all.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (RemoteHost :> sub) -> OpenApi

(ToSchema a, AllAccept cs, HasOpenApi sub, KnownSymbol (FoldDescription mods)) => HasOpenApi (ReqBody' mods cs a :> sub :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (ReqBody' mods cs a :> sub) -> OpenApi

(ToSchema a, Accept ct, HasOpenApi sub, KnownSymbol (FoldDescription mods)) => HasOpenApi (StreamBody' mods fr ct a :> sub :: Type)

This instance is an approximation.

Since: 1.1.7

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (StreamBody' mods fr ct a :> sub) -> OpenApi

(ToSchema a, HasStatus a, AllAccept cs, OpenApiMethod method, HasOpenApi (UVerb method cs as)) => HasOpenApi (UVerb method cs (a ': as) :: Type)

@since 2.0.1.0

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (UVerb method cs (a ': as)) -> OpenApi

HasOpenApi (UVerb method cs ('[] :: [Type]) :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (UVerb method cs '[]) -> OpenApi

HasOpenApi sub => HasOpenApi (WithNamedContext x c sub :: Type)

WithNamedContext combinator does not change our specification at all.

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (WithNamedContext x c sub) -> OpenApi

(AllAccept cs, KnownNat status, OpenApiMethod method) => HasOpenApi (Verb method status cs NoContent :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Verb method status cs NoContent) -> OpenApi

(AllAccept cs, AllToResponseHeader hs, KnownNat status, OpenApiMethod method) => HasOpenApi (Verb method status cs (Headers hs NoContent) :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Verb method status cs (Headers hs NoContent)) -> OpenApi

(ToSchema a, AllAccept cs, AllToResponseHeader hs, KnownNat status, OpenApiMethod method) => HasOpenApi (Verb method status cs (Headers hs a) :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Verb method status cs (Headers hs a)) -> OpenApi

(ToSchema a, AllAccept cs, KnownNat status, OpenApiMethod method) => HasOpenApi (Verb method status cs a :: Type)

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Verb method status cs a) -> OpenApi

(ToSchema a, Accept ct, KnownNat status, OpenApiMethod method) => HasOpenApi (Stream method status fr ct a :: Type)

Since: 1.1.7

Defined in Servant.OpenApi.Internal

Methods

toOpenApi :: Proxy (Stream method status fr ct a) -> OpenApi

Manipulation

subOperations

Arguments

:: (IsSubAPI sub api, HasOpenApi sub)

=> Proxy sub

Part of a servant API.

-> Proxy api

The whole servant API.

-> Traversal' OpenApi Operation

All operations of sub API. This is similar to operationsOf but ensures that operations indeed belong to the API at compile time.

Testing

validateEveryToJSON

Arguments

:: forall proxy api. TMap (Every [Typeable, Show, Arbitrary, ToJSON, ToSchema]) (BodyTypes JSON api)

=> proxy api

Servant API.

-> Spec

Verify that every type used with JSON content type in a servant API has compatible ToJSON and ToSchema instances using validateToJSON.

NOTE: validateEveryToJSON does not perform string pattern validation. See validateEveryToJSONWithPatternChecker.

validateEveryToJSON will produce one prop specification for every type in the API. Each type only gets one test, even if it occurs multiple times in the API.

>>> data User = User { name :: String, age :: Maybe Int } deriving (Show, Generic, Typeable)
>>> newtype UserId = UserId String deriving (Show, Generic, Typeable, ToJSON, Arbitrary)
>>> instance ToJSON User
>>> instance ToSchema User
>>> instance ToSchema UserId
>>> instance Arbitrary User where arbitrary = User <$> arbitrary <*> arbitrary
>>> type UserAPI = (Capture "user_id" UserId :> Get '[JSON] User) :<|> (ReqBody '[JSON] User :> Post '[JSON] UserId)

>>> hspec $ context "ToJSON matches ToSchema" $ validateEveryToJSON (Proxy :: Proxy UserAPI)

ToJSON matches ToSchema
  User...
...
  UserId...
...
Finished in ... seconds
...2 examples, 0 failures...

For the test to compile all body types should have the following instances:

• ToJSON and ToSchema are used to perform the validation;
• Typeable is used to name the test for each type;
• Show is used to display value for which ToJSON does not satisfy ToSchema.
• Arbitrary is used to arbitrarily generate values.

If any of the instances is missing, you'll get a descriptive type error:

>>> data Contact = Contact { fullname :: String, phone :: Integer } deriving (Show, Generic)
>>> instance ToJSON Contact
>>> instance ToSchema Contact
>>> type ContactAPI = Get '[JSON] Contact
>>> hspec $ validateEveryToJSON (Proxy :: Proxy ContactAPI)
...
...No instance for (Arbitrary Contact)
...  arising from a use of ‘validateEveryToJSON’
...

validateEveryToJSONWithPatternChecker

Arguments

:: forall proxy api. TMap (Every [Typeable, Show, Arbitrary, ToJSON, ToSchema]) (BodyTypes JSON api)

=> (Pattern -> Text -> Bool)

Pattern checker.

-> proxy api

Servant API.

-> Spec

Verify that every type used with JSON content type in a servant API has compatible ToJSON and ToSchema instances using validateToJSONWithPatternChecker.

For validation without patterns see validateEveryToJSON.