Safe Haskell	Safe-Inferred
Language	Haskell2010

Data.String.Compat

Synopsis

module Data.String
type String = [Char]
lines :: String -> [String]
words :: String -> [String]
unlines :: [String] -> String
unwords :: [String] -> String

Documentation

module Data.String

type String = [Char] Source #

String is an alias for a list of characters.

String constants in Haskell are values of type String. That means if you write a string literal like "hello world", it will have the type [Char], which is the same as String.

Note: You can ask the compiler to automatically infer different types with the -XOverloadedStrings language extension, for example "hello world" :: Text. See IsString for more information.

Because String is just a list of characters, you can use normal list functions to do basic string manipulation. See Data.List for operations on lists.

Performance considerations

Expand

[Char] is a relatively memory-inefficient type. It is a linked list of boxed word-size characters, internally it looks something like:

╭─────┬───┬──╮  ╭─────┬───┬──╮  ╭─────┬───┬──╮  ╭────╮
│ (:) │   │ ─┼─>│ (:) │   │ ─┼─>│ (:) │   │ ─┼─>│ [] │
╰─────┴─┼─┴──╯  ╰─────┴─┼─┴──╯  ╰─────┴─┼─┴──╯  ╰────╯
        v               v               v
       'a'             'b'             'c'

The String "abc" will use 5*3+1 = 16 (in general 5n+1) words of space in memory.

Furthermore, operations like (++) (string concatenation) are O(n) (in the left argument).

For historical reasons, the base library uses String in a lot of places for the conceptual simplicity, but library code dealing with user-data should use the text package for Unicode text, or the the bytestring package for binary data.

lines :: String -> [String] Source #

Splits the argument into a list of lines stripped of their terminating n characters. The n terminator is optional in a final non-empty line of the argument string.

For example:

>>> lines ""           -- empty input contains no lines
[]

>>> lines "\n"         -- single empty line
[""]

>>> lines "one"        -- single unterminated line
["one"]

>>> lines "one\n"      -- single non-empty line
["one"]

>>> lines "one\n\n"    -- second line is empty
["one",""]

>>> lines "one\ntwo"   -- second line is unterminated
["one","two"]

>>> lines "one\ntwo\n" -- two non-empty lines
["one","two"]

When the argument string is empty, or ends in a n character, it can be recovered by passing the result of lines to the unlines function. Otherwise, unlines appends the missing terminating n. This makes unlines . lines idempotent:

(unlines . lines) . (unlines . lines) = (unlines . lines)

words :: String -> [String] Source #

words breaks a string up into a list of words, which were delimited by white space.

>>> words "Lorem ipsum\ndolor"
["Lorem","ipsum","dolor"]

unlines :: [String] -> String Source #

Appends a n character to each input string, then concatenates the results. Equivalent to foldMap (s -> s ++ "n").

>>> unlines ["Hello", "World", "!"]
"Hello\nWorld\n!\n"

Note

unlines . lines /= id when the input is not n-terminated:

>>> unlines . lines $ "foo\nbar"
"foo\nbar\n"

unwords :: [String] -> String Source #

unwords is an inverse operation to words. It joins words with separating spaces.

>>> unwords ["Lorem", "ipsum", "dolor"]
"Lorem ipsum dolor"