Snap-Predicates
===============

This library provides a definition of a type-class `Predicate`
together with several concrete implementations which are used to
constrain the set of possible `Snap` handlers in a type-safe
way.

Example
-------

```haskell
{-# LANGUAGE OverloadedStrings, TypeOperators #-}
module Main where

import Data.ByteString (ByteString)
import Data.Monoid
import Data.Predicate
import Snap.Core
import Snap.Routes
import Snap.Predicates
import Snap.Http.Server

main :: IO ()
main = do
    mapM_ putStrLn (showRoutes sitemap)
    quickHttpServe (route . expandRoutes $ sitemap)

sitemap :: Routes Snap ()
sitemap = do
    get "/a" getUser $
        AcceptJson :&: (Param "name" :|: Param "nick") :&: Param "foo"

    get "/b" getUser' $
        AcceptJson :&: (Param "name" :||: Param "nick") :&: Param "foo"

    get  "/status" status      $ Fail (410, Just "Gone.")
    post "/"       createUser  $ AcceptThrift
    post "/"       createUser' $ AcceptJson

getUser :: AcceptJson :*: ByteString :*: ByteString -> Snap ()
getUser (_ :*: name :*: foo) =
    writeBS $ "getUser: name or nick=" <> name <> " foo=" <> foo

getUser' :: AcceptJson :*: (ByteString :+: ByteString) :*: ByteString -> Snap ()
getUser' (_ :*: name :*: foo) =
    case name of
        Left  a -> writeBS $ "getUser: name=" <> a <> " foo=" <> foo
        Right b -> writeBS $ "getUser: nick=" <> b <> " foo=" <> foo

createUser :: AcceptThrift -> Snap ()
createUser _ = writeBS "createUser"

createUser' :: AcceptJson -> Snap ()
createUser' _ = writeBS "createUser'"

status :: AcceptJson :*: Char -> Snap ()
status _ = writeBS "status"
```