A csv reader based on Haskell-cassava library : performance

I implemented my own csv reader using cassava library. The reader from missingh library was taking too long (~ 17 seconds) for a file with 43200 lines. I compared the result with python-numpy and python-pandas csv reader. Below is rough comparison.

cassava (ignore #) 3.3 sec
cassava (no support for ignoring #) 2.7 sec
numpy loadtxt > 10 sec
pandas read_csv 1.5 sec

As obvious, pandas does really well at reading csv file. I was hoping that my csv reader would do better but it didn’t. But it still beats the parsec based reader hands down.

The code is here https://github.com/dilawar/HBatteries/blob/master/src/HBatteries/CSV.hs

Computing value of pi using Monte-Carlo sampling

I used Haskell to do this. Code is here I did not use standard System.Random library to generate random number but mwc-random package. I can’t say if speed is better or worse.

I wanted to see how fast we converge to the value of pi. On x-axis, we have number of samples (N), and for each N, we compute the value of pi 50 times. This way we can talk about mean and variance in computed pi (which we plot on y-axis). It seems that for N >= 1000, we get a reasonable approximation of pi with low variance.

For a 2-D sphere (aka circle: x_1^2 + x_2 ^ 2 = 1 ), The convergence is following.


For a 3-D sphere, (that is x_1 ^2 + x_2 ^2 + x_3 ^3 = 1), I get the following:


Now I am bit more ambitious, I use a 10-D sphere:

As you can see, we converge to a a reasonable value of pi with low variance but convergence is not improving monotonically as in previous cases. The mean starts from a very low value and get to a respectable value only after N > 1000 (in low-dimensions cases we were swinging around 3.1415). The answer: we need to sample minimum number of points from hyper-cube to get some reasonable estimate.


How variance decreases with N (when N is significantly large, 100 repeats of each computation)? Following plots starts from 2D and goes onto 10 dimensions.


Generating random numbers in haskell (fast)

import qualified System.Random.MWC as R 
import Data.Vector.Unboxed as U 

-- A random numbers generator (vector from a uniform distribution). 
random_floats :: Int -> IO (U.Vector Float) 
random_floats n = R.createSystemRandom >>= \gen -> R.uniformVector gen n 
-- To scale the random floats in (0, 1.0) to (min, max)
-- y = (max - min) x + min 
 scale_samples :: (Float, Float) -> U.Vector Float -> U.Vector Float 
 scale_samples (min, max) vs = U.map (\v -> (max - min) * v + min ) vs 
 -- Sample a given space for n points. 
 sampled_space :: [( Float, Float)] -> Int -> IO [ U.Vector Float ] 
 sampled_space axes n = 
 random_floats n >>= \vv -> return $ Prelude.map (\x -> scale_samples x vv ) axes
main = do 
    vs <- sampled_space [ (1,3), (-1,1) ] 100 
    print vs 
    putStrLn $ "Done"

Hangman in Haskell

I wrote a Haskell implementation of a popular game Hangman. Hangman thinks of a word and tells you its length. You get that many chance to guess it. The procedure of guessing is following assuming length of the word is n.

  • You guess a letter, if the word contains that letter, hangman reveals the location of this letter. You still have n more guesses.
  • If the word does not contain your letter, your no of guesses is reduced by 1.
  • If you guess the word before n == 0, you live else you die.

You have to get two files from this folder; hangman.hs which contains the code and words which contains all possible words hangman can guess. Compile it using ghc compiler.

ghc --make hangman.hs

Run the binary produced. Have fun.

Lobbying for Haskell

… but one must not be surprised if outsiders don’t take Haskell too seriously… [We] say a lot about what functional programming isn’t: It has no assignment, no side effect, no flow of control but not much about what it is. The functional programmer sounds rather a medieval monk, denying himself the pleasures of life in hope that it will make him virtuous. To those interested in material benefits, these ‘advantages’ are totally unconvincing.

— “Why functional programming matters”

John Hughes, The Computer Journal, 1989

If someone comes to you with a new programming language, it is the time to run in the other direction. Who needs one more language; don’t we have millions of them already? Besides, if I can do something in a language I know well with the help of libraries, should I invest my time in learning a new language? A young student does not have such worldly concerns. He can be easily excited about a new language.

Why functional programming language? And why Haskell? Computer science people have their own reasons on why they prefer one language over other. But what about others, especially Electrical Engineering people, who can be equally, and often are, equally concerned about languages.

Electrical engineers are usually deals with hardware and the way things gets executed on it. They prefer C/C++ to explore their hardware and rightfully so. I doubt if any language can beat it on ‘fetch and execute’ type of processors (except for assembly). There is also designing and modeling side of electrical engineering. Here we have HDLs, and some other languages. Python is a versatile language designed for convenience: no seg-faults and core dumps. Moving from C to Python is relatively easy. Both languages have same type of flow. C/C++ is often taught in all engineering branches; python can be learned with little effort. Its an incredibly good combination.

In functional programming languages, most if not all things are functions. A function takes functions as argument and return a function as result. In C/C++, to do this, you have to use function pointers imaginatively. It’s a loose definition which might irritate those who have a theoretically neat mind. Nonetheless, it will serve our purpose for this informal discussion. It’s the ease with which we can ‘glue’ functions together to create a new function which makes functional programming attractive. You can go through a classic work, ‘Structure and Interpretation of Computer Programs’ to get the taste of functional programming. They have used Scheme, a dialect of Lisp language which is a functional programming language, old as Fortran. Moving from Lisp or Scheme to Haskell is rather simple. But learning Haskell can be challenging for those who are from C world. Many of them usually give up when they are confronted with monad.

We can list out some benefits of Haskell over C/C++. The cost is speed and memory. I am not suggesting that Haskell is always slow. It can be very fast, and sometimes as fast as C. Both Python and Haskell drops into C to do computationally expensive tasks. While there is little you can do in pure Python to make things faster, Haskell provides many opportunities for optimization at its back-end. Learning to optimize the Haskell code can be challenging. A lot of work has been done on these directions.

The beauty of Haskell (or any functional programming language) lies in that fact that it captures the idea of recursion most naturally. Once you know the syntax, writing recursive functions are extremely easy. This is usually their selling point: recursive functions and recursive data-structures are easier to handle. Python has also borrowed some of these functional programming features such as list-comprehension and lambda functions (anonymous function). The C++ 2011 standard has Lambda function. That should be the proof enough of its syntactic beauty.

The core of Haskell is small. If lambda calculus — which is Turning complete — has only two operations and can model any computation then why should core of any language have more? Clean syntax and a small core are beneficial for those who infer the logic by looking at the program. Translation of Haskell code to something else should be easier. Moreover as parallel hardwares are becoming common-place, functional languages will increase in their stature. Figuring out parallel component in C/C++ is not easy. Programmer has to figure it out and code it by himself. People have found that it is relatively much easier for Haskell compiler to figure the parallelism out by itself.

If you have ever written a large program (say a thousand line of code) then you must have spent more time debugging the application than coding it. In fact, a saying in programming community goes, ‘Anyone can code, it takes a programmer to debug’. Haskell reduces chances of a programmer creating bugs significantly by its strong type system. In Electrical Engineering, no one really gives a damn if a language has strong type-system. Who cares if VHDL have a stronger type-system than Verilog? You should ask your CS friend about it and its merit. A simple example might throw some light on it.

typedef speed_t double;
typedef acceleration_t double;
typedef distance_t double;

speed_t a =10.0;
acceleration_t b
distance_t c
= a + b;

Now it is illegal to add speed and acceleration. But it is a legal piece of code in C. Such bugs will be incredibly hard to locate in C. An equivalent code in Haskell would not compile at all. The operator ‘+’ demands that a and b should be of same type which they are not. Since Haskell check types at compile type (unlike python which checks it at run-time), it will easily catch it (and can easily irritate you).

People talk about many other benefits Haskell provides to a programmer. One of them is that one has to type fewer lines. This is certainly true but what is doubtful if producing n line of code in Haskell is less time consuming that writing its equivalent in Python.

My own experience with Haskell language are of mixed kind.  When you are working with recursive stuff, Haskell is very pleasant to code in. When it comes to non-recursive stuff such as graph, Haskell can become painfully slow. It has to convert graph (which is a non-recursive data-structure) to an equivalent recursive structure which takes a lot of memory. You will overflow your stack more often in Haskell than in Python or C. Hardcore algorithmic people who deals with large graphs will not find it very exciting.

I’d recommend Haskell not because you are going to use to use it in your professional life. Most probably you won’t. It would be a worthy investment just to learn the most rudimentary Haskell. Haskell makes you think and reason at very high level. Mistakes made at high levels become easily apparent in Haskell then they would be in C or Python. Moreover, once you have become efficient in Haskell, breaking your design in smaller pieces and gluing them together will come naturally to you. If writing modular program is a virtue than Haskell is perhaps one of the best language around. Breaking and gluing functions is the essence of of functional programming; it is made easier in Haskell. Learning Haskell will make you a better programmer even if you are not coding in Haskell.

You can also try Scheme. Best thing about Scheme is that you can call yourself a ‘schemer’ who schemes everyday, which is million times cooler than calling yourself a ‘Haskeller’.

Resources :

[1] Prof. Sanyal page on Haskell. Check out the problem sheet.
[2] Real world Haskell. This is a really good book to start with.
[3] http://learnyouahaskell.com/ . Read it at your own risk. Sometimes he says much about nothing.
[4] Structure and interpretation of computer program. This old classic is freely available. Still a great book to learn about computer programs.


Quine McClusky algorithm in Haskell

A small program tingu is implemented in haskell. It reads minterms from a file and produce the output on console. It is hosted on github. You can use it like

 ./tingu -i minterms 

Your minterms should be described in a file like following

 vars = 4
minterms = 1,2,5,7,8,9,10,13,15, 

Then output of program is

 Hello, I am Tingu, the mighty crab! And I like tea. You should call me $./tingu -i filename from your terminal. 
** I have found the essential prime implicants of your function. 
** Verifying my answers. It may take some time.... 
+++ OK, passed 100 tests. Peace!! 

Peace !!

Python like string split in Haskell

Following function split a string into a list of strings at a given string. For example split "dilawar raw war" "aw" returns ["dil", "ar r" " war"]

split :: String -> String -> [String]
split str pat = helper str pat [] [] where 
    helper :: String -> String -> String -> String -> [String]
    helper [] ys n m = [n] ++ []
    helper xs [] n m = [n] ++ (split xs pat)
    helper (x:xs) (y:ys) n m
        | x /= y = helper (xs) pat ((n++m)++[x]) m
        | otherwise = helper xs ys n (m++[y])