Cookbook/Lists and strings

Lists

In Haskell, lists are what Arrays are in most other languages.

Creating simple lists

Problem	Solution	Examples
creating a list with given elements	-	3 : 12 : 42 : [] --> [3,12,42] 'f' : 'o' : 'o' : [] --> "foo"
creating a list with stepsize 1	-	[1..10] --> [1,2,3,4,5,6,7,8,9,10] ['a'..'z'] --> "abcdefghijklmnopqrstuvwxyz"
creating a list with different stepsize	-	[1,3..10] --> [1,3,5,7,9] ['a','c'..'z'] --> "acegikmoqsuwy"
creating an infinite constant list	-	[1,1..] --> [1,1,1,1,1,...
creating an infinite list with stepsize 1	-	[1..] --> [1,2,3,4,5,...

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 lists	(++)	"foo" ++ "bar" --> "foobar" [42,43] ++ [60,61] --> [42,43,60,61]
combining many lists	concat	concat ["foo", "bar", "baz"] --> "foobarbaz"

Accessing sublists

Problem	Solution	Examples
accessing the first element	head	head "foo bar baz" --> 'f'
accessing the last element	last	last "foo bar baz" --> 'z'
accessing the element at a given index	(!!)	"foo bar baz" !! 4 --> 'b'
accessing the first n elements	take	take 3 "foo bar baz" --> "foo"
accessing the last n elements	reverse , take	reverse . take 3 . reverse $ "foobar" --> "bar"
accessing the n elements 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 list into two parts	splitAt	splitAt 3 "foo bar baz" --> ("foo"," bar baz")

Strings

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

Multiline strings

"foo\
\bar"               --> "foobar"

Converting between characters and values

Problem	Solution	Examples
converting a character to a numeric value	ord	import Data.Char ord 'A' --> 65
converting a numeric value to a character	chr	import Data.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 Data.Char toUpper 'a' --> 'A'
converting a character to lower-case	toLower	import Data.Char toLower 'A' --> 'a'
converting a string to upper-case	toUpper, map	import Data.Char map toUpper "Foo Bar" --> "FOO BAR"
converting a string to lower-case	toLower, map	import Data.Char map toLower "Foo Bar" --> "foo bar"

Interpolation

TODO

Performance

Text handles character strings with better performance than Strings; it should be the prefered data type for UTF-8 encoded strings.

If observe that Text does not give sufficient performance, consider Data.ByteString, which is essentially a byte array. It can contain UTF-8 characters, but handle with care! .

Unicode

Current GHC (later than 6) encodes Strings and Text in UTF-8. This may change the behavior of some of the functions explained above when applied to characters beyond the traditional ASCII characters. Remember that not every character in UTF-8 encoding is one byte!