The fn package

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A Haskell web framework where you write plain old functions.

Provided you have stack installed, you can run this example like a shell script:

#!/usr/bin/env stack
-- stack --resolver lts-3.10 --install-ghc runghc --package fn
{-# LANGUAGE OverloadedStrings #-}
import Data.Monoid ((<>))
import Data.Text (Text)
import Network.Wai (Response)
import Network.Wai.Handler.Warp (run)
import Web.Fn

data Ctxt = Ctxt { _req :: FnRequest }
instance RequestContext Ctxt where
  getRequest = _req
  setRequest c r = c { _req = r }

initializer :: IO Ctxt
initializer = return (Ctxt defaultFnRequest)

main :: IO ()
main = do ctxt <- initializer
          run 3000 $ toWAI ctxt site

site :: Ctxt -> IO Response
site ctxt = route ctxt [ end                        ==> indexH
                       , path "echo" /? param "msg" ==> echoH
                       , path "echo" // segment     ==> echoH
                  `fallthrough` notFoundText "Page not found."

indexH :: Ctxt -> IO (Maybe Response)
indexH _ = okText "Try visiting /echo?msg=hello or /echo/hello"

echoH :: Ctxt -> Text -> IO (Maybe Response)
echoH _ msg = okText $ "Echoing '" <> msg <> "'."

Fn lets you write web code that just looks like normal Haskell code.

The name comes from the fact that Fn emphasizes functions (over monads), where all necessary data is passed via function arguments, and control flow is mediated by return values.

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Dependenciesbase (>=4.7 && <5), blaze-builder, bytestring, directory, filepath, http-types, text, unordered-containers, wai (>=3), wai-extra (>=3) [details]
Copyright2015 Daniel Patterson
AuthorDaniel Patterson <>
Home page
Source repositoryhead: git clone
UploadedWed Jan 20 14:30:51 UTC 2016 by DanielPatterson




Maintainers' corner

For package maintainers and hackage trustees

Readme for fn-

Fn (eff-enn) - a functional web framework.

Or, how to do away with the monad transformers, and just use plain functions.


See the example application in the repository for a full usage including database access, heist templates, sessions, etc.