A practice: implementing a web service which receives a number by URL parameter and returns the corresponding Fibonacci sequence number.
GET
http://localhost:8000/fib/10
returns
fib(10) = 55
I used Snap Framework.
$ mkdir snapfib
$ cd snapfib
$ snap init
This generates an empty snap project files. then
$ cabal install
$ snapfib
builds the codes and installs a command snapfib
in your ~/.cabal/bin/snapfib
. You may run the app locally with the default port 8000; you may open the default page, hello world, on your browser.
Then implement fib sequence, a utility function, and the controller/view. This allows you to get the Fibonacci sequence number just by URL parameter.
Below is the main routine code from the repository.
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Applicative
import Snap.Types
import Snap.Util.FileServe
import Text.Templating.Heist
import Text.Templating.Heist.TemplateDirectory
import Glue
import Server
import Data.ByteString.Char8 (pack, unpack)
fibs :: [Integer]
fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
fibInString :: Int -> String
fibInString n = "fib(" ++ show n ++ ") = " ++ show (fibs !! n)
main :: IO ()
main = do
td <- newTemplateDirectory' "templates" emptyTemplateState
quickServer $ templateHandler td defaultReloadHandler $ \ts ->
ifTop (writeBS "hello world") <|>
route [ ("foo", writeBS "bar")
, ("fib/:n", fibHandler)
] <|>
templateServe ts <|>
dir "static" (fileServe ".")
fibHandler :: Snap ()
fibHandler = do
param <- getParam "n"
maybe (writeBS "must specify fib/n in URL")
(writeBS . pack . fibInString . read . unpack) param
(writeBS . pack . fibInString . read . unpack) param
is converting param :: Maybe ByteString
into Int
, getting a Fibonacci sequence number in String, converting into ByteString again, and passing it to writeBS
function which is defined in Snap Framework.
It was not difficult or complicated to implement such a simple web service in Haskell as long as you have basic Haskell knowledges like Maybe Monad or String manipulations. The problem was, in my opinion, it took long time to build the web app. Every time you fix your code, you have to wait for the compilation before you access your web service.
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