Haskell list comprehension on variable number of strings

Question

If I have two strings I use a list comprehension to obtain the desired result:

 combineStrings firstStr sndStr = [ [a,b] | a <- firstStr, b <- sndStr]

For three strings, I use this

 combineStrings firstStr sndStr trdStr = [ [a,b,c] | a <- firstStr, b <- sndStr, c <- trdStr]

What I'm trying is to obtain the same result for a variable number of strings. For example if I have a function which takes the following form:

 combineStrings :: [String] -> [String]

I'm trying to obtain the same results as above for 2, 3 ... n lists... I tried multiple ways, like this one

 combineStrings []      = []
 combineStrings (hd:tl) = [ a:b | a <- hd, b <- combineStrings tl]

but this fails because of [] on the first clause. Can someone help me to write this, please?

As a general rule, you can try to make types as general as possible. Here your code is not specific to strings, and can be used for all lists, so you can write combineStrings :: [[a]] -> [[a]]. Then, the first line from undur_gongor's answer can be written combineStrings [] = [[]]. You can also try to write combineStrings using foldr, as it fits the fold pattern. — sdcvvc
– sdcvvc, Commented Jan 4, 2012 at 14:33

Daniel Fischer · Accepted Answer · 2012-01-04 14:26:53Z

15

Noteworthy: Haskell already has that function, just a bit more general:

Prelude> :t sequence
sequence :: Monad m => [m a] -> m [a]
Prelude> sequence ["ab","cd","12"]
["ac1","ac2","ad1","ad2","bc1","bc2","bd1","bd2"]

[] is an instance of Monad, so in this case the signature becomes sequence :: [[a]] -> [[a]], with a = Char, sequence :: [String] -> [String].

answered Jan 4, 2012 at 14:26

Daniel Fischer

184k19 gold badges319 silver badges436 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Adi Over a year ago

I wonder how many functions like this are re-written by newcomers like me.. But it's a learning experience :) Thank you

Daniel Fischer Over a year ago

Many. And it's a good thing, as long as they're told soon enough that there's a (more general, usually) function in the standard libraries already, because, as you said, it's a very good learning experience. Just, once you've figured out how it works by writing your own version, use the library function other Haskellers already know.

nponeccop Over a year ago

Hoogle and HLint are there to help you.

Luis Casillas Over a year ago

@Adi @DanielFischer And actually, at this point, a good exercise would be to write one's own implementation of sequence. These simple-but-very-general functions always make good learning exercises.

michael_wu · Accepted Answer · 2012-01-04 14:58:21Z

9

Try

combineStrings [] = [""]

or better (as pointed out by sdcwc):

combineStrings [] = [[]]

Otherwise the part b <- combineStrings tl of the list comprehension will not yield any b and you will always end up with an empty array.

It also makes sense as an edge case: The only way to combine characters from zero strings is an empty string (consisting of zero characters).

edited Jan 4, 2012 at 14:58

answered Jan 4, 2012 at 14:12

michael_wu

16k5 gold badges66 silver badges77 bronze badges

Collectives™ on Stack Overflow

Haskell list comprehension on variable number of strings

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related