String

A built-in representation for efficient string manipulation. String literals are enclosed in "double quotes". Strings are not lists of characters.

Basics

isEmpty : String -> Bool

Determine if a string is empty.

isEmpty "" == True
isEmpty "the world" == False

length : String -> Int

Get the length of a string.

length "innumerable" == 11
length "" == 0

reverse : String -> String

Reverse a string.

reverse "stressed" == "desserts"

repeat : Int -> String -> String

Repeat a string n times.

repeat 3 "ha" == "hahaha"

Building and Splitting

cons : Char -> String -> String

Add a character to the beginning of a string.

cons 'T' "he truth is out there" == "The truth is out there"

uncons : String -> Maybe ( Char, String )

Split a non-empty string into its head and tail. This lets you pattern match on strings exactly as you would with lists.

uncons "abc" == Just ('a',"bc")
uncons ""    == Nothing

fromChar : Char -> String

Create a string from a given character.

fromChar 'a' == "a"

append : String -> String -> String

Append two strings. You can also use the (++) operator to do this.

append "butter" "fly" == "butterfly"

concat : List String -> String

Concatenate many strings into one.

concat ["never","the","less"] == "nevertheless"

split : String -> String -> List String

Split a string using a given separator.

split "," "cat,dog,cow"        == ["cat","dog","cow"]
split "/" "home/evan/Desktop/" == ["home","evan","Desktop", ""]

Use Regex.split if you need something more flexible.

join : String -> List String -> String

Put many strings together with a given separator.

join "a" ["H","w","ii","n"]        == "Hawaiian"
join " " ["cat","dog","cow"]       == "cat dog cow"
join "/" ["home","evan","Desktop"] == "home/evan/Desktop"

words : String -> List String

Break a string into words, splitting on chunks of whitespace.

words "How are \t you? \n Good?" == ["How","are","you?","Good?"]

lines : String -> List String

Break a string into lines, splitting on newlines.

lines "How are you?\nGood?" == ["How are you?", "Good?"]

Get Substrings

slice : Int -> Int -> String -> String

Take a substring given a start and end index. Negative indexes are taken starting from the end of the list.

slice  7  9 "snakes on a plane!" == "on"
slice  0  6 "snakes on a plane!" == "snakes"
slice  0 -7 "snakes on a plane!" == "snakes on a"
slice -6 -1 "snakes on a plane!" == "plane"

left : Int -> String -> String

Take n characters from the left side of a string.

left 2 "Mulder" == "Mu"

right : Int -> String -> String

Take n characters from the right side of a string.

right 2 "Scully" == "ly"

dropLeft : Int -> String -> String

Drop n characters from the left side of a string.

dropLeft 2 "The Lone Gunmen" == "e Lone Gunmen"

dropRight : Int -> String -> String

Drop n characters from the right side of a string.

dropRight 2 "Cigarette Smoking Man" == "Cigarette Smoking M"

Check for Substrings

contains : String -> String -> Bool

See if the second string contains the first one.

contains "the" "theory" == True
contains "hat" "theory" == False
contains "THE" "theory" == False

Use Regex.contains if you need something more flexible.

startsWith : String -> String -> Bool

See if the second string starts with the first one.

startsWith "the" "theory" == True
startsWith "ory" "theory" == False

endsWith : String -> String -> Bool

See if the second string ends with the first one.

endsWith "the" "theory" == False
endsWith "ory" "theory" == True

indexes : String -> String -> List Int

Get all of the indexes for a substring in another string.

indexes "i" "Mississippi"   == [1,4,7,10]
indexes "ss" "Mississippi"  == [2,5]
indexes "needle" "haystack" == []

indices : String -> String -> List Int

Alias for indexes.

Conversions

toInt : String -> Result String Int

Try to convert a string into an int, failing on improperly formatted strings.

String.toInt "123" == Ok 123
String.toInt "-42" == Ok -42
String.toInt "3.1" == Err "could not convert string '3.1' to an Int"
String.toInt "31a" == Err "could not convert string '31a' to an Int"

If you are extracting a number from some raw user input, you will typically want to use Result.withDefault to handle bad data:

Result.withDefault 0 (String.toInt "42") == 42
Result.withDefault 0 (String.toInt "ab") == 0

toFloat : String -> Result String Float

Try to convert a string into a float, failing on improperly formatted strings.

String.toFloat "123" == Ok 123.0
String.toFloat "-42" == Ok -42.0
String.toFloat "3.1" == Ok 3.1
String.toFloat "31a" == Err "could not convert string '31a' to a Float"

If you are extracting a number from some raw user input, you will typically want to use Result.withDefault to handle bad data:

Result.withDefault 0 (String.toFloat "42.5") == 42.5
Result.withDefault 0 (String.toFloat "cats") == 0

toList : String -> List Char

Convert a string to a list of characters.

toList "abc" == ['a','b','c']

fromList : List Char -> String

Convert a list of characters into a String. Can be useful if you want to create a string primarily by consing, perhaps for decoding something.

fromList ['a','b','c'] == "abc"

Formatting

Cosmetic operations such as padding with extra characters or trimming whitespace.

toUpper : String -> String

Convert a string to all upper case. Useful for case-insensitive comparisons and VIRTUAL YELLING.

toUpper "skinner" == "SKINNER"

toLower : String -> String

Convert a string to all lower case. Useful for case-insensitive comparisons.

toLower "X-FILES" == "x-files"

pad : Int -> Char -> String -> String

Pad a string on both sides until it has a given length.

pad 5 ' ' "1"   == "  1  "
pad 5 ' ' "11"  == "  11 "
pad 5 ' ' "121" == " 121 "

padLeft : Int -> Char -> String -> String

Pad a string on the left until it has a given length.

padLeft 5 '.' "1"   == "....1"
padLeft 5 '.' "11"  == "...11"
padLeft 5 '.' "121" == "..121"

padRight : Int -> Char -> String -> String

Pad a string on the right until it has a given length.

padRight 5 '.' "1"   == "1...."
padRight 5 '.' "11"  == "11..."
padRight 5 '.' "121" == "121.."

trim : String -> String

Get rid of whitespace on both sides of a string.

trim "  hats  \n" == "hats"

trimLeft : String -> String

Get rid of whitespace on the left of a string.

trimLeft "  hats  \n" == "hats  \n"

trimRight : String -> String

Get rid of whitespace on the right of a string.

trimRight "  hats  \n" == "  hats"

Higher-Order Functions

map : (Char -> Char) -> String -> String

Transform every character in a string

map (\c -> if c == '/' then '.' else c) "a/b/c" == "a.b.c"

filter : (Char -> Bool) -> String -> String

Keep only the characters that satisfy the predicate.

filter isDigit "R2-D2" == "22"

foldl : (Char -> b -> b) -> b -> String -> b

Reduce a string from the left.

foldl cons "" "time" == "emit"

foldr : (Char -> b -> b) -> b -> String -> b

Reduce a string from the right.

foldr cons "" "time" == "time"

any : (Char -> Bool) -> String -> Bool

Determine whether any characters satisfy a predicate.

any isDigit "90210" == True
any isDigit "R2-D2" == True
any isDigit "heart" == False

all : (Char -> Bool) -> String -> Bool

Determine whether all characters satisfy a predicate.

all isDigit "90210" == True
all isDigit "R2-D2" == False
all isDigit "heart" == False

elm-lang/core/4.0.5