A middleware is an indexed monadic action transforming one Conn to another Conn. It operates in some base monad m, and is indexed by i and o, the input and output Conn types of the middleware action.

newtype Middleware m i o a = ...

The input and output type parameters are used to ensure that a Conn is transformed, and that side-effects are performed, correctly, throughout the middleware chain.

Being able to parameterize Middleware with some type m, you can customize the chain depending on the needs of your middleware and handlers. Applications can use monad transformers to track state, provide configuration, gather metrics, and much more, in the chain of middleware.

Middleware are composed using ibind, the indexed monadic version of bind. The simplest way of composing middleware is by chaining them with :*>, from Control.IxMonad. See haskell-indexed-monad for more information.

writeStatus statusOK
:*> closeHeaders
:*> respond "We're composing middleware!"

If you want to feed the return value of one middleware into another, use :>>=, the infix operator alias for mbind.

getUser :>>= renderUser

You can also rebind the do block syntax to use ibind instead of regular bind.

  user <- getUser
  writeStatus statusOK
  respond ("User: " <> user.name)
  where bind = ibind