Difference between revisions of "Cookbook/Lists and strings"

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= Lists =
+
== Lists ==
   
In Haskell, lists are what Arrays are in most other languages. Haskell has all of the general list manipulation functions, see also <hask>Data.List</hask>.
+
In Haskell, lists are what Arrays are in most other languages.
   
 
=== Creating simple lists ===
<haskell>
 
head [1,2,3] --> 1
 
tail [1,2,3] --> [2,3]
 
length [1,2,3] --> 3
 
init [1,2,3] --> [1,2]
 
last [1,2,3] --> 3
 
</haskell>
 
   
  +
{| class="wikitable"
Furthermore, Haskell supports some neat concepts.
 
  +
|-
 
  +
! Problem
== Infinite lists ==
 
  +
! Solution
<haskell>
 
  +
! Examples
Prelude> [1..]
 
  +
|-
  +
| creating a list with given elements
  +
| -
 
|<haskell>
  +
3 : 12 : 42 : [] --> [3,12,42]
  +
'f' : 'o' : 'o' : [] --> "foo"
 
</haskell>
 
</haskell>
  +
|-
 
  +
| creating a list with stepsize 1
The list of all squares:
 
  +
| -
<haskell>
 
 
|<haskell>
square x = x*x
 
  +
[1..10] --> [1,2,3,4,5,6,7,8,9,10]
squares = map square [1..]
 
  +
['a'..'z'] --> "abcdefghijklmnopqrstuvwxyz"
 
</haskell>
 
</haskell>
  +
|-
 
  +
| creating a list with different stepsize
But in the end, you probably don't want to use infinite lists, but make them finite. You can do this with <hask>take</hask>:
 
  +
| -
 
<haskell>
+
|<haskell>
 
[1,3..10] --> [1,3,5,7,9]
Prelude> take 10 squares
 
  +
['a','c'..'z'] --> "acegikmoqsuwy"
[1,4,9,16,25,36,49,64,81,100]
 
 
</haskell>
 
</haskell>
  +
|-
  +
| creating an infinite constant list
  +
| -
 
|<haskell>
  +
[1,1..] --> [1,1,1,1,1,...
 
</haskell>
  +
|-
  +
| creating an infinite list with stepsize 1
  +
| -
  +
| <haskell>
  +
[1..] --> [1,2,3,4,5,...
  +
</haskell>
  +
|}
   
== List comprehensions ==
+
=== List comprehensions ===
   
 
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
 
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
Line 47: Line 61:
   
   
== Combining lists ==
+
=== Combining lists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 56: Line 70:
 
|-
 
|-
 
| combining two lists
 
| combining two lists
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A%2B%2B (++)]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3A%2B%2B (++)]
 
|<haskell>
 
|<haskell>
 
"foo" ++ "bar" --> "foobar"
 
"foo" ++ "bar" --> "foobar"
  +
[42,43] ++ [60,61] --> [42,43,60,61]
 
</haskell>
 
</haskell>
 
|-
 
|-
 
| combining many lists
 
| combining many lists
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:concat concat]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:concat concat]
 
| <haskell>
 
| <haskell>
 
concat ["foo", "bar", "baz"] --> "foobarbaz"
 
concat ["foo", "bar", "baz"] --> "foobarbaz"
Line 68: Line 83:
 
|}
 
|}
   
== Accessing sublists ==
+
=== Accessing sublists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 76: Line 91:
 
! Examples
 
! Examples
 
|-
 
|-
| accessing the first character
+
| accessing the first element
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:head head]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:head head]
 
|<haskell>
 
|<haskell>
 
head "foo bar baz" --> 'f'
 
head "foo bar baz" --> 'f'
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the last character
+
| accessing the last element
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Alast last]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Alast last]
 
|<haskell>
 
|<haskell>
 
last "foo bar baz" --> 'z'
 
last "foo bar baz" --> 'z'
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the character at a given index
+
| accessing the element at a given index
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A!! (!!)]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3A!! (!!)]
 
|<haskell>
 
|<haskell>
 
"foo bar baz" !! 4 --> 'b'
 
"foo bar baz" !! 4 --> 'b'
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the first <code>n</code> characters
+
| accessing the first <code>n</code> elements
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:take take]
 
| <haskell>
 
| <haskell>
 
take 3 "foo bar baz" --> "foo"
 
take 3 "foo bar baz" --> "foo"
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the last <code>n</code> characters
+
| accessing the last <code>n</code> elements
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse ], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse ], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:take take]
 
| <haskell>
 
| <haskell>
 
reverse . take 3 . reverse $ "foobar" --> "bar"
 
reverse . take 3 . reverse $ "foobar" --> "bar"
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the <code>n</code> characters starting from index <code>m</code>
+
| accessing the <code>n</code> elements starting from index <code>m</code>
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:take take]
 
| <haskell>
 
| <haskell>
 
take 4 $ drop 2 "foo bar baz" --> "o ba"
 
take 4 $ drop 2 "foo bar baz" --> "o ba"
Line 113: Line 128:
 
|}
 
|}
   
== Splitting lists ==
+
=== Splitting lists ===
   
   
Line 123: Line 138:
 
|-
 
|-
 
| splitting a string into a list of words
 
| splitting a string into a list of words
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words words]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:words words]
 
| <haskell>words "foo bar\t baz\n" --> ["foo","bar","baz"]
 
| <haskell>words "foo bar\t baz\n" --> ["foo","bar","baz"]
 
</haskell>
 
</haskell>
 
|-
 
|-
| splitting a string into two parts
+
| splitting a list into two parts
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AsplitAt splitAt]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3AsplitAt splitAt]
 
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz")
 
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz")
 
</haskell>
 
</haskell>
 
|}
 
|}
   
= Strings =
+
== Strings ==
   
 
Since strings are lists of characters, you can use any available list function.
 
Since strings are lists of characters, you can use any available list function.
   
== Multiline strings ==
+
=== Multiline strings ===
 
<haskell>
 
<haskell>
 
"foo\
 
"foo\
Line 143: Line 158:
 
</haskell>
 
</haskell>
   
== Converting between characters and values ==
+
=== Converting between characters and values ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 152: Line 167:
 
|-
 
|-
 
| converting a character to a numeric value
 
| converting a character to a numeric value
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v:ord ord]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v:ord ord]
 
|<haskell>
 
|<haskell>
import Char
+
import Data.Char
 
ord 'A' --> 65
 
ord 'A' --> 65
 
</haskell>
 
</haskell>
 
|-
 
|-
 
| converting a numeric value to a character
 
| converting a numeric value to a character
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3Achr chr]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3Achr chr]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
 
chr 99 --> 'c'
 
chr 99 --> 'c'
 
</haskell>
 
</haskell>
 
|}
 
|}
   
== Reversing a string by words or characters ==
+
=== Reversing a string by words or characters ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 175: Line 190:
 
|-
 
|-
 
| reversing a string by characters
 
| reversing a string by characters
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse]
 
|<haskell>
 
|<haskell>
 
reverse "foo bar baz" --> "zab rab oof"
 
reverse "foo bar baz" --> "zab rab oof"
Line 181: Line 196:
 
|-
 
|-
 
| reversing a string by words
 
| reversing a string by words
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aunwords unwords]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Aunwords unwords]
 
| <haskell>
 
| <haskell>
 
unwords $ reverse $ words "foo bar baz" --> "baz bar foo"
 
unwords $ reverse $ words "foo bar baz" --> "baz bar foo"
Line 187: Line 202:
 
|-
 
|-
 
| reversing a string by characters by words
 
| reversing a string by characters by words
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aunwords unwords]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Aunwords unwords]
 
| <haskell>
 
| <haskell>
 
unwords $ map reverse $ words "foo bar baz" --> "oof rab zab"
 
unwords $ map reverse $ words "foo bar baz" --> "oof rab zab"
Line 193: Line 208:
 
|}
 
|}
   
== Converting case ==
+
=== Converting case ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 202: Line 217:
 
|-
 
|-
 
| converting a character to upper-case
 
| converting a character to upper-case
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoUpper toUpper]
 
|<haskell>
 
|<haskell>
import Char
+
import Data.Char
toUpper 'a' --> "A"
+
toUpper 'a' --> 'A'
 
</haskell>
 
</haskell>
 
|-
 
|-
 
| converting a character to lower-case
 
| converting a character to lower-case
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoLower toLower]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
toLower 'A' --> "a"
+
toLower 'A' --> 'a'
 
</haskell>
 
</haskell>
 
|-
 
|-
 
| converting a string to upper-case
 
| converting a string to upper-case
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map]
 
|<haskell>
 
|<haskell>
import Char
+
import Data.Char
 
map toUpper "Foo Bar" --> "FOO BAR"
 
map toUpper "Foo Bar" --> "FOO BAR"
 
</haskell>
 
</haskell>
 
|-
 
|-
 
| converting a string to lower-case
 
| converting a string to lower-case
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
 
map toLower "Foo Bar" --> "foo bar"
 
map toLower "Foo Bar" --> "foo bar"
 
</haskell>
 
</haskell>
 
|}
 
|}
   
== Interpolation ==
+
=== Interpolation ===
   
 
TODO
 
TODO
   
== Performance ==
+
=== Performance ===
   
  +
Text handles character strings with better performance than Strings; it should be the prefered data type for UTF-8 encoded strings.
For high performance requirements (where you would typically consider
 
C), consider using [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString].
 
   
 
If observe that Text does not give sufficient performance, consider [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString], which is essentially a byte array. It can contain UTF-8 characters, but handle with care! .
== Unicode ==
 
   
 
=== Unicode ===
TODO
 
  +
  +
Current GHC (later than 6) encodes Strings and Text in UTF-8. This may change the behavior of some of the functions explained above when applied to characters beyond the traditional ASCII characters. Remember that not every character in UTF-8 encoding is one byte!

Latest revision as of 21:02, 6 January 2019

Lists

In Haskell, lists are what Arrays are in most other languages.

Creating simple lists

Problem Solution Examples
creating a list with given elements -
3 : 12 : 42 : []        --> [3,12,42]
'f' : 'o' : 'o' : []    --> "foo"
creating a list with stepsize 1 -
[1..10]                 --> [1,2,3,4,5,6,7,8,9,10]
['a'..'z']              --> "abcdefghijklmnopqrstuvwxyz"
creating a list with different stepsize -
[1,3..10]               --> [1,3,5,7,9]
['a','c'..'z']          --> "acegikmoqsuwy"
creating an infinite constant list -
[1,1..]                   --> [1,1,1,1,1,...
creating an infinite list with stepsize 1 -
[1..]                 --> [1,2,3,4,5,...

List comprehensions

The list of all squares can also be written in a more comprehensive way, using list comprehensions:

squares = [x*x | x <- [1..]]

List comprehensions allow for constraints as well:

-- multiples of 3 or 5
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]


Combining lists

Problem Solution Examples
combining two lists (++)
"foo" ++ "bar"                  --> "foobar"
[42,43] ++ [60,61]              --> [42,43,60,61]
combining many lists concat
concat ["foo", "bar", "baz"]    --> "foobarbaz"

Accessing sublists

Problem Solution Examples
accessing the first element head
head "foo bar baz"      --> 'f'
accessing the last element last
last "foo bar baz"      --> 'z'
accessing the element at a given index (!!)
"foo bar baz" !! 4      --> 'b'
accessing the first n elements take
take 3 "foo bar baz"    --> "foo"
accessing the last n elements reverse , take
reverse . take 3 . reverse $ "foobar"    --> "bar"
accessing the n elements starting from index m drop, take
take 4 $ drop 2 "foo bar baz"            --> "o ba"

Splitting lists

Problem Solution Examples
splitting a string into a list of words words
words "foo bar\t baz\n"    --> ["foo","bar","baz"]
splitting a list into two parts splitAt
splitAt 3 "foo bar baz"    --> ("foo"," bar baz")

Strings

Since strings are lists of characters, you can use any available list function.

Multiline strings

"foo\
\bar"               --> "foobar"

Converting between characters and values

Problem Solution Examples
converting a character to a numeric value ord
import Data.Char
ord 'A'    --> 65
converting a numeric value to a character chr
import Data.Char
chr 99     --> 'c'

Reversing a string by words or characters

Problem Solution Examples
reversing a string by characters reverse
reverse "foo bar baz"                        --> "zab rab oof"
reversing a string by words words, reverse, unwords
unwords $ reverse $ words "foo bar baz"      --> "baz bar foo"
reversing a string by characters by words words, reverse, map, unwords
unwords $ map reverse $ words "foo bar baz"  --> "oof rab zab"

Converting case

Problem Solution Examples
converting a character to upper-case toUpper
import Data.Char
toUpper 'a'            --> 'A'
converting a character to lower-case toLower
import Data.Char
toLower 'A'            --> 'a'
converting a string to upper-case toUpper, map
import Data.Char
map toUpper "Foo Bar"  --> "FOO BAR"
converting a string to lower-case toLower, map
import Data.Char
map toLower "Foo Bar"  --> "foo bar"

Interpolation

TODO

Performance

Text handles character strings with better performance than Strings; it should be the prefered data type for UTF-8 encoded strings.

If observe that Text does not give sufficient performance, consider Data.ByteString, which is essentially a byte array. It can contain UTF-8 characters, but handle with care! .

Unicode

Current GHC (later than 6) encodes Strings and Text in UTF-8. This may change the behavior of some of the functions explained above when applied to characters beyond the traditional ASCII characters. Remember that not every character in UTF-8 encoding is one byte!