base-4.20.2.0: Core data structures and operations

Copyright | (c) The University of Glasgow 2007
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | stable
Portability | portable
Safe Haskell | Safe
Language | Haskell2010

Data.String

Contents

- Functions on strings

Description

The `String` type and associated operations.

Synopsis

- type String = [Char]
- class IsString a where
  - fromString :: String -> a
- lines :: String -> [String]
- words :: String -> [String]
- unlines :: [String] -> String
- unwords :: [String] -> String

# Documentation

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.

class IsString a where Source #

`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.

Methods

fromString :: String -> a Source #

#### Instances

Instances details

IsString a => IsString (Identity a) | Since: base-4.9.0.0
Instance details Defined in GHC.Internal.Data.String Methods fromString :: String -> Identity a Source #
a ~ Char => IsString [a] | `(a ~ Char)` context was introduced in `4.9.0.0` Since: base-2.1
Instance details Defined in GHC.Internal.Data.String Methods fromString :: String -> [a] Source #
IsString a => IsString (Const a b) | Since: base-4.9.0.0
Instance details Defined in GHC.Internal.Data.String Methods fromString :: String -> Const a b Source #

# Functions on strings

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.

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

Expand

    >>> 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 :: String -> [String] Source #

`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

Expand

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

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

unlines :: [String] -> String Source #

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

#### Examples

Expand

    >>> 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 :: [String] -> String Source #

`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

Expand

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

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