Text

A built-in representation for efficient string manipulation. Text values are not lists of characters.

Text

type Text = Text

A Text is a chunk of text:

"Hello!"
"How are you?"
"🙈🙉🙊"

-- strings with escape characters
"this\n\t\"that\""
"\x1F648\x1F649\x1F64A" -- "🙈🙉🙊"

A Text can represent any sequence of unicode characters. You can use the unicode escapes from x0000 to x10FFFF to represent characters by their code point. You can also include the unicode characters directly. Using the escapes can be better if you need one of the many whitespace characters with different widths.

isEmpty :: Text -> Bool

Determine if a string is empty.

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

length :: Text -> Int

Get the length of a string.

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

reverse :: Text -> Text

Reverse a string.

reverse "stressed" == "desserts"

repeat :: Int -> Text -> Text

Repeat a string n times.

repeat 3 "ha" == "hahaha"

replace :: Text -> Text -> Text -> Text

Replace all occurrences of some substring.

replace "." "-" "Json.Decode.succeed" == "Json-Decode-succeed"
replace "," "/" "a,b,c,d,e"           == "a/b/c/d/e"

Building and Splitting

append :: Text -> Text -> Text

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

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

concat :: List Text -> Text

Concatenate many strings into one.

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

split :: Text -> Text -> List Text

Split a string using a given separator.

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

join :: Text -> List Text -> Text

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 :: Text -> List Text

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

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

lines :: Text -> List Text

Break a string into lines, splitting on newlines.

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

Get Substrings

slice :: Int -> Int -> Text -> Text

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 -> Text -> Text

Take n characters from the left side of a string.

left 2 "Mulder" == "Mu"

right :: Int -> Text -> Text

Take n characters from the right side of a string.

right 2 "Scully" == "ly"

dropLeft :: Int -> Text -> Text

Drop n characters from the left side of a string.

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

dropRight :: Int -> Text -> Text

Drop n characters from the right side of a string.

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

Check for Substrings

contains :: Text -> Text -> Bool

See if the second string contains the first one.

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

startsWith :: Text -> Text -> Bool

See if the second string starts with the first one.

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

endsWith :: Text -> Text -> Bool

See if the second string ends with the first one.

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

indexes :: Text -> Text -> 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 :: Text -> Text -> List Int

Alias for indexes.

Int Conversions

toInt :: Text -> Maybe Int

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

Text.toInt "123" == Just 123
Text.toInt "-42" == Just -42
Text.toInt "3.1" == Nothing
Text.toInt "31a" == Nothing

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

Maybe.withDefault 0 (Text.toInt "42") == 42
Maybe.withDefault 0 (Text.toInt "ab") == 0

fromInt :: Int -> Text

Convert an Int to a Text.

Text.fromInt 123 == "123"
Text.fromInt -42 == "-42"

Float Conversions

toFloat :: Text -> Maybe Float

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

Text.toFloat "123" == Just 123.0
Text.toFloat "-42" == Just -42.0
Text.toFloat "3.1" == Just 3.1
Text.toFloat "31a" == Nothing

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

Maybe.withDefault 0 (Text.toFloat "42.5") == 42.5
Maybe.withDefault 0 (Text.toFloat "cats") == 0

fromFloat :: Float -> Text

Convert a Float to a Text.

Text.fromFloat 123 == "123"
Text.fromFloat -42 == "-42"
Text.fromFloat 3.9 == "3.9"

Char Conversions

fromChar :: Char -> Text

Create a Text from a given character.

fromChar 'a' == "a"

cons :: Char -> Text -> Text

Add a character to the beginning of a Text.

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

uncons :: Text -> Maybe (Char, Text)

Split a non-empty Text 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

List Conversions

toList :: Text -> List Char

Convert a Text to a list of characters.

toList "abc" == ['a','b','c']
toList "🙈🙉🙊" == ['🙈','🙉','🙊']

fromList :: List Char -> Text

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

fromList ['a','b','c'] == "abc"
fromList ['🙈','🙉','🙊'] == "🙈🙉🙊"

Formatting

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

toUpper :: Text -> Text

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

toUpper "skinner" == "SKINNER"

toLower :: Text -> Text

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

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

pad :: Int -> Char -> Text -> Text

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 -> Text -> Text

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 -> Text -> Text

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 :: Text -> Text

Get rid of whitespace on both sides of a string.

trim "  hats  \n" == "hats"

trimLeft :: Text -> Text

Get rid of whitespace on the left of a string.

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

trimRight :: Text -> Text

Get rid of whitespace on the right of a string.

trimRight "  hats  \n" == "  hats"

Higher-Order Functions

map :: (Char -> Char) -> Text -> Text

Transform every character in a Text

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

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

Keep only the characters that pass the test.

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

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

Reduce a Text from the left.

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

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

Reduce a Text from the right.

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

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

Determine whether any characters pass the test.

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

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

Determine whether all characters pass the test.

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