Haskell on Lorenzo Bolla

URL Counter in GO

Wed, 10 Oct 2012 00:00:00 +0000

After having tinkered with #haskell for quite a bit, I decided that I needed some rest from theory and esoteric concepts, and a more pragmatic programming language to explore.

I've spent the last few days refreshing my memories on Go: I hadn’t touched it for almost 2 years and I must say that I find it changed: for the better.

Here is a short tutorial on how to write a simple web application in Go, and publish it on Google App Engine. The application is not a mere exercise, but scratches an itch I recently had: it counts how many times each of its handlers is hit. So, for example, visiting: http://go-count-urls.appspot.com/hello returns how many times the /hello handler has been visited. You can use it as a trivial real-time tracker.

Tornado vs. Warp vs. Yesod

Sun, 02 Sep 2012 00:00:00 +0000

Today I tried to benchmark 3 web servers that I've used recently:

In fact, these are only *2* web servers, because Yesod runs on top of Warp and it's a fully fledged web framework, rather than a web server: but this was also the intent of the benchmark, i.e. to measure how slower all its goodies made Yesod with respect to Warp.

Tornado and Warp are obviously very different web servers (async vs. threaded, interpreted vs. compiled, etc.) but, who cares?

hsenv

Thu, 29 Mar 2012 00:00:00 +0000

If you have never used hsenv before, you should. It's basically the Haskell's equivalent of Python's virtualenv.

You can find a tweaked version of hsenv here: I've relaxed some requirements in order to make it install on newer GHC releases.

Finally, if when using you hit this error:

$ hsenv Creating Virtual Haskell directory structure
Installing GHC Initializing GHC Package database at /home/lollo/work/Unique/.hsenv/ghc_pkg_db
Copying necessary packages from original GHC package database
hsenv: fd:9: hGetContents: invalid argument (invalid byte sequence)
hsenv: thread blocked indefinitely in an MVar operation

Then, you have a problem with your locale. Follow the steps here, and retry.

Sudoku solver in Haskell

Mon, 27 Feb 2012 00:00:00 +0000

Recently, I've been challenged to write a Sudoku solver. Not knowing the rules and with only 30 minutes, I needed some help…

So, in case someone asks me again, I've implemented the same algorithm in Haskell.

I know that there are already a lot of other solutions, but, hey, it was fun!

ulint, Universal Lint

Thu, 05 Jan 2012 00:00:00 +0000

I've been using so many code static checkers lately, that I decided to write a wrapper around the ones I use the most. I called it: Universal Lint, ulint for short.

At the moment, Python, Javascript and Haskell files are supported, but adding new linters/extension is trivial.

Download it and use it!

HSGrep benchmarking

Wed, 30 Nov 2011 00:00:00 +0000

Few days ago, I rewrote sgrep in Haskell. I was curious to know how it compares to grep in term of execution speed. In particular, I was interested to verify that hsgrep scales as O(log n), instead of O(n), with n being the size of the file analyzed.

First of all, in order to have similar performance to grep, I had to convert my original program to use Haskell's bytestrings. You can find the code here. Testing files are generated with this script.

HSGrep Sorted grep in Haskell

Sun, 27 Nov 2011 00:00:00 +0000

As an exercise to learn Haskell, I wrote a specialized grep to work on sorted files. It uses binary search to scan a text file and print all the (consecutive) lines that start with a user defined string.

My program is a rewrite of sgrep in Haskell: I called it HSGrep. Code is available on github.

Thanks a lot for all your useful suggestions! As soon as possible, I'll post some benchmarking here. (EDIT: benchmarks now available!)

After downloading the source code, build install and run it with:

Haskell and VIM

Sat, 15 Oct 2011 00:00:00 +0000

I had to tinker quite a bit before finding a decent configuration for vim to edit Haskell files. Here are the packages and config files I use:

Haskell mode: interaction with Haddock, compiler integration, and other general settings.
Indentation: proper source code indentation.
More configs to enforce good style (no tabs, tabstops, etc.):

You can also have them bundled all together.

HTTP client in Haskell

Mon, 26 Sep 2011 00:00:00 +0000

To try to make sense of this, I decided to write a "simple" http client in Haskell in as many styles as I could think of:

import Network.HTTP
import Control.Applicative

url = "http://www.haskell.org/haskellwiki/Haskell"

-- Imperative style
fetch_1 = do
 rsp <- Network.HTTP.simpleHTTP (getRequest url)
 body <- getResponseBody rsp
 return (take 1000 body)

-- With Functors' fmap
fetch_2 = do
 rsp <- Network.HTTP.simpleHTTP (getRequest url)
 fmap (take 1000) (getResponseBody rsp)

-- With Applicative's >>=
fetch_3 = fmap (take 1000) (Network.HTTP.simpleHTTP (getRequest url) >>= getResponseBody)

-- "fmap f x" is the same as "pure f <*> x" (Applicative's law)
fetch_4 = pure (take 1000) <*> (Network.HTTP.simpleHTTP (getRequest url) >>= getResponseBody)

-- "pure f <*> x" is the same as "f <$> x"
fetch_5 = (take 1000) <$> (Network.HTTP.simpleHTTP (getRequest url) >>= getResponseBody)

It's a long time since I found so many new concepts while studying a new programming language…