A suite of Haskell packages for HTTP servers and clients with typesafe REST interfaces.

The central contribution of these packages are the typed representation of REST resources, and machinery that uses these resource types to guide the generation of clients that can request them and servers that can serve them.

This means translating HTTP URIs to haskell types and generating corresponding client functions for requesting resources and server middleware for serving resources as modeled by the types.

The state of these packages is highly experimental, and the documentation assumes that the reader is comfortable with the DataKinds extension and type families.

Given a REST interface to a web store with /store/inventory representing the current storage inventory of the shop, for our purposes encoded as a JSON list of item objects. And assume that /store/inventory/<item> refers to the inventory status of a particular item identified by an id number in place of <item>.

In our setting, this would be represented by the types (with imports and instance declarations censored for ease of display):

import Network.HTTP.Rest.Signature

data StoreItem = StoreItem {...}

inventoryRes = RestResource
  :: RestSig
    (S "store" :/: S "inventory" :/: Nil)
    ('HttpGet [StoreItem])

inventoryItemRes = RestResource
  :: RestSig
    (S "store" :/: S "inventory" :/: A "itemNumber" Int :/: Nil)
    ('HttpGet [StoreItem])

Now inventoryRes represents the API of the inventory resource. As such, the name inventoryRes is interesting only inasmuch as it is a witness of its type, which is used to drive instance selection when synthesizing client functions or server middleware.

To actually use the inventoryRes (for example as a client), we rely on the RequestResource typeclass (here as a ghci session) :

> :set -XOverloadedStrings
> :set -XTypeOperators
> :set -XDataKinds
> import Network.HTTP.Rest.Client

> :t requestResourceDef "example.com" inventoryRes
requestResource "example.com" inventoryRes
  :: RequestPathSig (S "store" :/: S "inventory") (RequestMethodSig ('HttpGet [StoreItem])

> :kind! RequestPathSig (S "store" :/: S "inventory") (RequestMethodSig ('HttpGet [StoreItem])
RequestPathSig (S "store" :/: S "inventory") (RequestMethodSig ('HttpGet [StoreItem]) :: *
= IO [StoreItem]

> :t requestResourceDef "example.com" inventoryItemRes
> :kind! RequestPathSig (S "store" :/: S "inventory" :/: A "itemNumber" Int :/: Nil) (RequestMethodSig ('HttpGet [StoreItem])
RequestPathSig (S "store" :/: S "inventory" :/: A "itemNumber" Int :/: Nil) (RequestMethodSig ('HttpGet [StoreItem]) :: *
= Int -> IO [StoreItem]

The requestResourceDef function construct requestor actions given a hostname and a RestSig. The type of a requestor function is computed by the associated type families RequestPathSig and RequestMethodSig. The resulting types are accessible to the programmer by means of the :kind! ghci command.

Informally, the resulting type of RequestPathsig can be described as taking as arguments each of the 'A "variable" a parts of a http path (i.e. ignoring 'S "..." components), ending with the type of the request payload and resulting in IO responesType (All determined by RequestMethodSig. Example:

> :kind! RequestPathSig
    (A "foo" Int :/: S "bar" :/: A "baz" (Maybe Bool) :/: Nil)
    (RequestMethodSig (HttpPost String Int))
RequestPathSig
  (A "foo" Int :/: S "bar" :/: A "baz" (Maybe Bool) :/: Nil)
  (RequestMethodSig (HttpPost String Int)) :: *
= Int -> Maybe Bool -> String -> IO Int

(TODO!)

NixOS nix-expressions are provided for each package, along with a project-global default.nix that just wires up each package independently. No attempt has yet been made for integration with the existing haskell packaging infrastructure in NixOS.

Regular linux distributions (or maybe even entirely different operating systems) should just be able to install via cabal, with the caveat that this is untested by the package author, and I cannot guarantee any support. However, I would be very surprised if this was out of reach for a moderately experienced cabal user.

Notifications on observed upper and lower bounds different from those recorded in the cabal files though (as well as other bugs in general) is greatly appreaciated.