Data.String

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

[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.

IsString is used in combination with the -XOverloadedStrings language extension to convert the literals to different string types.

For example, if you use the text package, you can say

{-# LANGUAGE OverloadedStrings  #-}

myText = "hello world" :: Text

Internally, the extension will convert this to the equivalent of

myText = fromString @Text ("hello world" :: String)

Note: You can use fromString in normal code as well, but the usual performance/memory efficiency problems with String apply.

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.

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)

Examples

>>> 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"]

words breaks a string up into a list of words, which were delimited by white space (as defined by isSpace). This function trims any white spaces at the beginning and at the end.

Examples

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

>>> words " foo bar "["foo","bar"]

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

Examples

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

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

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

unwords joins words with separating spaces (U+0020 SPACE).

unwords is neither left nor right inverse of words:

>>> words (unwords [" "])[]>>> unwords (words "foo\nbar")"foo bar"

Examples

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

>>> unwords ["foo", "bar", "", "baz"]"foo bar  baz"