a newbie does list comprehensions

Formatting this post in WordPress.com was a great pain. It does not render correctly on some browser / device combinations, despite my rewrite efforts. So a Markdown copy of this post can be found as a gist here.


The year is 1998 and @mtheofy then at Glasgow tells me about a relatively new (then) language called Haskell. I’m intrigued but do not do much. A few years later I buy The Haskell School of Expression since The Craft of Functional Programming did not seem enough to motivate me. Time passes and around 2007 I try yet another start. Nothing. I promised my self yet another restart for a 2017 new year’s resolution. Still nothing. So when the current employer offered Haskell classes I could not say no. Armed with the weekly classes and a Safari Learning Path I am trying to correct this. And I am having some fun with list comprehensions. Because as a friend says, if it makes you feel good, go.

So how do you write an infinite list? Let’s say you want list x to include all numbers from 0 to infinity. stack ghci is my friend. Others might try repl.it:

x = [ n | n <- [0..]]

Now you can have the first 20 items of x:

Prelude> x = [ n | n <- [0..]]
Prelude> take 20 x
[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
Prelude>

So next I wanted to make an infinite list of the same character. Enter the underscore variable:

Prelude> x = [ 'a' | _ <- [0..]]
Prelude> take 20 x
"aaaaaaaaaaaaaaaaaaaa"
Prelude>

OK, so now let’s try to cycle infinitely characters from a string. I end up with:

Prelude> x = [ c | i  take 20 x
"abcdabcdabcdabcdabcd"
Prelude>

I am kind of unsure why the let statements are needed since I am ~10 days into typing stuff and posted my creation to twitter. What my expression says is that x is comprised of characters from string “abcd”, where given a sequence of numbers, each time a character is chosen based on the sequence number modulo 4. Strings are lists of characters in Haskell and list indexing starts from zero.  Helpful comments come my way. Like the obvious cycle (there is a cycle function? Yes ):

Prelude> take 20 (cycle "abcd")
"abcdabcdabcdabcdabcd"
Prelude> take 20 $ cycle "abcd"
"abcdabcdabcdabcdabcd"
Prelude>

Is not the dollar operator nice to get rid of parentheses? Here is another suggestion about cycling a string:

Prelude> x = [ "abcd" !! (i `mod` 4) | i  take 20 x
"abcdabcdabcdabcdabcd"
Prelude>

This one is more concise and does the same thing, always picking a character from "abcd". If the infix notation for mod confuses you, you can:

Prelude> x = [ "abcd" !! (mod i 4) | i  take 20 x
"abcdabcdabcdabcdabcd"
Prelude>

But the Internet does not stop there. It comes back with more helpful suggestions:

Welcome! A little feedback then if I may: the !! operator should be used VERY cautiously it is not typesafe and lists are not random access anyway. Opt for a function returning Maybe x and for a random access datastructure (strings are by default lists).

Which made me think: How about an infinite string randomly chosen from “abcd”?

$ stack install random
$ stack ghci
:
Prelude> import System.Random
Prelude System.Random> g <- newStdGen 
Prelude System.Random> x = [ "abcd" !! i | i <- randomRs (0,3) g ]
Prelude System.Random> take 10 x
"bcbbddcdab"
Prelude System.Random>

If you want a sequence with a different order, you need to reinitialise both g and x. I will figure out a better way some other time when …I have time.

Adventures with Maybe maybe in another post.

Formatting this post in WordPress.com was a great pain.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s