Cookbook/Lists and strings

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Since strings are lists of characters, you can use any available list function.


In Haskell, lists are what Arrays are in most other languages. Haskell has all of the general list manipulation functions, see also Data.List.

head [1,2,3]      --> 1
tail [1,2,3]      --> [2,3]
length [1,2,3]    --> 3
init [1,2,3]      --> [1,2]
last [1,2,3]      --> 3

Furthermore, Haskell supports some neat concepts.

Infinite lists

Prelude> [1..]

The list of all squares:

square x = x*x
squares = map square [1..]

But in the end, you probably don't want to use infinite lists, but make them finite. You can do this with take:

Prelude> take 10 squares

List comprehensions

The list of all squares can also be written in a more comprehensive way, using list comprehensions:

squares = [x*x | x <- [1..]]

List comprehensions allow for constraints as well:

-- multiples of 3 or 5
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]

Combining lists

Problem Solution Examples
combining two strings (++)
"foo" ++ "bar"                  --> "foobar"
combining many strings concat
concat ["foo", "bar", "baz"]    --> "foobarbaz"

Accessing sublists

Problem Solution Examples
accessing the first character head
head "foo bar baz"      --> 'f'
accessing the last character last
last "foo bar baz"      --> 'z'
accessing the character at a given index (!!)
"foo bar baz" !! 4      --> 'b'
accessing the first n characters take
take 3 "foo bar baz"    --> "foo"
accessing the last n characters reverse , take
reverse . take 3 . reverse $ "foobar"    --> "bar"
accessing the n characters starting from index m drop, take
take 4 $ drop 2 "foo bar baz"            --> "o ba"

Splitting lists

Problem Solution Examples
splitting a string into a list of words words
words "foo bar\t baz\n"    --> ["foo","bar","baz"]
splitting a string into two parts splitAt
splitAt 3 "foo bar baz"    --> ("foo"," bar baz")


Multiline strings

\bar"               --> "foobar"

Converting between characters and values

Problem Solution Examples
converting a character to a numeric value ord
import Char
ord 'A'    --> 65
converting a numeric value to a character chr
import Char
chr 99     --> 'c'

Reversing a string by words or characters

Problem Solution Examples
reversing a string by characters reverse
reverse "foo bar baz"                        --> "zab rab oof"
reversing a string by words words, reverse, unwords
unwords $ reverse $ words "foo bar baz"      --> "baz bar foo"
reversing a string by characters by words words, reverse, map, unwords
unwords $ map reverse $ words "foo bar baz"  --> "oof rab zab"

Converting case

Problem Solution Examples
converting a character to upper-case toUpper
import Char
toUpper 'a'            --> "A"
converting a character to lower-case toLower
import Char
toLower 'A'            --> "a"
converting a string to upper-case toUpper, map
import Char
map toUpper "Foo Bar"  --> "FOO BAR"
converting a string to lower-case toLower, map
import Char
map toLower "Foo Bar"  --> "foo bar"




For high performance requirements (where you would typically consider C), consider using Data.ByteString.