| 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 | Trustworthy |
| Language | Haskell2010 |
Data.String
Contents
Description
The String type and associated operations.
Documentation
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.
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
| IsString a => IsString (Identity a) Source # | Since: base-4.9.0.0 |
Defined in Data.String Methods fromString :: String -> Identity a Source # | |
| a ~ Char => IsString [a] Source # |
Since: base-2.1 |
Defined in Data.String Methods fromString :: String -> [a] Source # | |
| IsString a => IsString (Const a b) Source # | Since: base-4.9.0.0 |
Defined in 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.
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"]