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.
fromString :: String -> a Source #
Instances
IsString a => IsString (Identity a) Source # | Since: base-4.9.0.0 |
Defined in Data.String fromString :: String -> Identity a Source # | |
a ~ Char => IsString [a] Source # |
Since: base-2.1 |
Defined in Data.String fromString :: String -> [a] Source # | |
IsString a => IsString (Const a b) Source # | Since: base-4.9.0.0 |
Defined in Data.String 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"]