import qualified Data.Text.Lazy as L

Most of the functions in this module are subject to fusion, meaning that a pipeline of such functions will usually allocate at most one Text value.

As an example, consider the following pipeline:

import Data.Text.Lazy as T
import Data.Text.Lazy.Encoding as E
import Data.ByteString.Lazy (ByteString)

countChars :: ByteString -> Int
countChars = T.length . T.toUpper . E.decodeUtf8

From the type signatures involved, this looks like it should allocate one ByteString value, and two Text values. However, when a module is compiled with optimisation enabled under GHC, the two intermediate Text values will be optimised away, and the function will be compiled down to a single loop over the source ByteString.

Functions that can be fused by the compiler are documented with the phrase "Subject to fusion".

A Text value is a sequence of Unicode scalar values, as defined in §3.9, definition D76 of the Unicode 5.2 standard. As such, a Text cannot contain values in the range U+D800 to U+DFFF inclusive. Haskell implementations admit all Unicode code points (§3.4, definition D10) as Char values, including code points from this invalid range. This means that there are some Char values that are not valid Unicode scalar values, and the functions in this module must handle those cases.

Within this module, many functions construct a Text from one or more Char values. Those functions will substitute Char values that are not valid Unicode scalar values with the replacement character "�" (U+FFFD). Functions that perform this inspection and replacement are documented with the phrase "Performs replacement on invalid scalar values".

(One reason for this policy of replacement is that internally, a Text value is represented as packed UTF-16 data. Values in the range U+D800 through U+DFFF are used by UTF-16 to denote surrogate code points, and so cannot be represented. The functions replace invalid scalar values, instead of dropping them, as a security measure. For details, see Unicode Technical Report 36, §3.5.)

data Text Source

pack :: String -> Text Source

O(n) Convert a String into a Text.

Subject to fusion. Performs replacement on invalid scalar values.

unpack :: Text -> String Source

O(n) Convert a Text into a String. Subject to fusion.

singleton :: Char -> Text Source

O(1) Convert a character into a Text. Subject to fusion. Performs replacement on invalid scalar values.

empty :: Text Source

Smart constructor for Empty.

fromChunks :: [Text] -> Text Source

O(c) Convert a list of strict Texts into a lazy Text.

toChunks :: Text -> [Text] Source

O(n) Convert a lazy Text into a list of strict Texts.

toStrict :: Text -> Text Source

O(n) Convert a lazy Text into a strict Text.

fromStrict :: Text -> Text Source

O(c) Convert a strict Text into a lazy Text.

foldrChunks :: (Text -> a -> a) -> a -> Text -> a Source

Consume the chunks of a lazy Text with a natural right fold.

foldlChunks :: (a -> Text -> a) -> a -> Text -> a Source

Consume the chunks of a lazy Text with a strict, tail-recursive, accumulating left fold.

cons :: Char -> Text -> Text infixr 5 Source

O(1) Adds a character to the front of a Text. Subject to fusion.

snoc :: Text -> Char -> Text Source

O(n) Adds a character to the end of a Text. This copies the entire array in the process, unless fused. Subject to fusion.

append :: Text -> Text -> Text Source

O(n/c) Appends one Text to another. Subject to fusion.

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

O(1) Returns the first character and rest of a Text, or Nothing if empty. Subject to fusion.

unsnoc :: Text -> Maybe (Text, Char) Source

O(n/c) Returns the init and last of a Text, or Nothing if empty.

• It is no faster than using init and last.

Since: text-1.2.3.0

head :: Text -> Char Source

O(1) Returns the first character of a Text, which must be non-empty. Subject to fusion.

last :: Text -> Char Source

O(n/c) Returns the last character of a Text, which must be non-empty. Subject to fusion.

tail :: Text -> Text Source

O(1) Returns all characters after the head of a Text, which must be non-empty. Subject to fusion.

init :: Text -> Text Source

O(n/c) Returns all but the last character of a Text, which must be non-empty. Subject to fusion.

null :: Text -> Bool Source

O(1) Tests whether a Text is empty or not. Subject to fusion.

length :: Text -> Int64 Source

O(n) Returns the number of characters in a Text. Subject to fusion.

compareLength :: Text -> Int64 -> Ordering Source

O(n) Compare the count of characters in a Text to a number. Subject to fusion.

This function gives the same answer as comparing against the result of length, but can short circuit if the count of characters is greater than the number, and hence be more efficient.

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

O(n) map f t is the Text obtained by applying f to each element of t. Subject to fusion. Performs replacement on invalid scalar values.

intercalate :: Text -> [Text] -> Text Source

O(n) The intercalate function takes a Text and a list of Texts and concatenates the list after interspersing the first argument between each element of the list.

intersperse :: Char -> Text -> Text Source

O(n) The intersperse function takes a character and places it between the characters of a Text. Subject to fusion. Performs replacement on invalid scalar values.

transpose :: [Text] -> [Text] Source

O(n) The transpose function transposes the rows and columns of its Text argument. Note that this function uses pack, unpack, and the list version of transpose, and is thus not very efficient.

reverse :: Text -> Text Source

O(n) reverse t returns the elements of t in reverse order.

replace Source

O(m+n) Replace every non-overlapping occurrence of needle in haystack with replacement.

This function behaves as though it was defined as follows:

replace needle replacement haystack =
  intercalate replacement (splitOn needle haystack)

As this suggests, each occurrence is replaced exactly once. So if needle occurs in replacement, that occurrence will not itself be replaced recursively:

replace "oo" "foo" "oo" == "foo"

In cases where several instances of needle overlap, only the first one will be replaced:

replace "ofo" "bar" "ofofo" == "barfo"

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

With Unicode text, it is incorrect to use combinators like map toUpper to case convert each character of a string individually. Instead, use the whole-string case conversion functions from this module. For correctness in different writing systems, these functions may map one input character to two or three output characters.

toCaseFold :: Text -> Text Source

O(n) Convert a string to folded case. Subject to fusion.

This function is mainly useful for performing caseless (or case insensitive) string comparisons.

A string x is a caseless match for a string y if and only if:

toCaseFold x == toCaseFold y

The result string may be longer than the input string, and may differ from applying toLower to the input string. For instance, the Armenian small ligature men now (U+FB13) is case folded to the bigram men now (U+0574 U+0576), while the micro sign (U+00B5) is case folded to the Greek small letter letter mu (U+03BC) instead of itself.

toLower :: Text -> Text Source

O(n) Convert a string to lower case, using simple case conversion. Subject to fusion.

The result string may be longer than the input string. For instance, the Latin capital letter I with dot above (U+0130) maps to the sequence Latin small letter i (U+0069) followed by combining dot above (U+0307).

toUpper :: Text -> Text Source

O(n) Convert a string to upper case, using simple case conversion. Subject to fusion.

The result string may be longer than the input string. For instance, the German eszett (U+00DF) maps to the two-letter sequence SS.

toTitle :: Text -> Text Source

O(n) Convert a string to title case, using simple case conversion. Subject to fusion.

The first letter of the input is converted to title case, as is every subsequent letter that immediately follows a non-letter. Every letter that immediately follows another letter is converted to lower case.

The result string may be longer than the input string. For example, the Latin small ligature fl (U+FB02) is converted to the sequence Latin capital letter F (U+0046) followed by Latin small letter l (U+006C).

Note: this function does not take language or culture specific rules into account. For instance, in English, different style guides disagree on whether the book name "The Hill of the Red Fox" is correctly title cased—but this function will capitalize every word.

Since: text-1.0.0.0

justifyLeft :: Int64 -> Char -> Text -> Text Source

O(n) Left-justify a string to the given length, using the specified fill character on the right. Subject to fusion. Performs replacement on invalid scalar values.

Examples:

justifyLeft 7 'x' "foo"    == "fooxxxx"
justifyLeft 3 'x' "foobar" == "foobar"

justifyRight :: Int64 -> Char -> Text -> Text Source

O(n) Right-justify a string to the given length, using the specified fill character on the left. Performs replacement on invalid scalar values.

Examples:

justifyRight 7 'x' "bar"    == "xxxxbar"
justifyRight 3 'x' "foobar" == "foobar"

center :: Int64 -> Char -> Text -> Text Source

O(n) Center a string to the given length, using the specified fill character on either side. Performs replacement on invalid scalar values.

Examples:

center 8 'x' "HS" = "xxxHSxxx"

foldl :: (a -> Char -> a) -> a -> Text -> a Source

O(n) foldl, applied to a binary operator, a starting value (typically the left-identity of the operator), and a Text, reduces the Text using the binary operator, from left to right. Subject to fusion.

foldl' :: (a -> Char -> a) -> a -> Text -> a Source

O(n) A strict version of foldl. Subject to fusion.

foldl1 :: (Char -> Char -> Char) -> Text -> Char Source

O(n) A variant of foldl that has no starting value argument, and thus must be applied to a non-empty Text. Subject to fusion.

foldl1' :: (Char -> Char -> Char) -> Text -> Char Source

O(n) A strict version of foldl1. Subject to fusion.

foldr :: (Char -> a -> a) -> a -> Text -> a Source

O(n) foldr, applied to a binary operator, a starting value (typically the right-identity of the operator), and a Text, reduces the Text using the binary operator, from right to left. Subject to fusion.

foldr1 :: (Char -> Char -> Char) -> Text -> Char Source

O(n) A variant of foldr that has no starting value argument, and thus must be applied to a non-empty Text. Subject to fusion.

concat :: [Text] -> Text Source

O(n) Concatenate a list of Texts.

concatMap :: (Char -> Text) -> Text -> Text Source

O(n) Map a function over a Text that results in a Text, and concatenate the results.

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

O(n) any p t determines whether any character in the Text t satisfies the predicate p. Subject to fusion.

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

O(n) all p t determines whether all characters in the Text t satisfy the predicate p. Subject to fusion.

maximum :: Text -> Char Source

O(n) maximum returns the maximum value from a Text, which must be non-empty. Subject to fusion.

minimum :: Text -> Char Source

O(n) minimum returns the minimum value from a Text, which must be non-empty. Subject to fusion.

scanl :: (Char -> Char -> Char) -> Char -> Text -> Text Source

O(n) scanl is similar to foldl, but returns a list of successive reduced values from the left. Subject to fusion. Performs replacement on invalid scalar values.

scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]

Note that

last (scanl f z xs) == foldl f z xs.

scanl1 :: (Char -> Char -> Char) -> Text -> Text Source

O(n) scanl1 is a variant of scanl that has no starting value argument. Performs replacement on invalid scalar values.

scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]

scanr :: (Char -> Char -> Char) -> Char -> Text -> Text Source

O(n) scanr is the right-to-left dual of scanl. Performs replacement on invalid scalar values.

scanr f v == reverse . scanl (flip f) v . reverse

scanr1 :: (Char -> Char -> Char) -> Text -> Text Source

O(n) scanr1 is a variant of scanr that has no starting value argument. Performs replacement on invalid scalar values.

mapAccumL :: (a -> Char -> (a, Char)) -> a -> Text -> (a, Text) Source

O(n) Like a combination of map and foldl'. Applies a function to each element of a Text, passing an accumulating parameter from left to right, and returns a final Text. Performs replacement on invalid scalar values.

mapAccumR :: (a -> Char -> (a, Char)) -> a -> Text -> (a, Text) Source

The mapAccumR function behaves like a combination of map and a strict foldr; it applies a function to each element of a Text, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new Text. Performs replacement on invalid scalar values.

repeat :: Char -> Text Source

repeat x is an infinite Text, with x the value of every element.

Since: text-1.2.0.5

replicate :: Int64 -> Text -> Text Source

O(n*m) replicate n t is a Text consisting of the input t repeated n times.

cycle :: Text -> Text Source

cycle ties a finite, non-empty Text into a circular one, or equivalently, the infinite repetition of the original Text.

Since: text-1.2.0.5

iterate :: (Char -> Char) -> Char -> Text Source

iterate f x returns an infinite Text of repeated applications of f to x:

iterate f x == [x, f x, f (f x), ...]

Since: text-1.2.0.5

unfoldr :: (a -> Maybe (Char, a)) -> a -> Text Source

O(n), where n is the length of the result. The unfoldr function is analogous to the List unfoldr. unfoldr builds a Text from a seed value. The function takes the element and returns Nothing if it is done producing the Text, otherwise Just (a,b). In this case, a is the next Char in the string, and b is the seed value for further production. Subject to fusion. Performs replacement on invalid scalar values.

unfoldrN :: Int64 -> (a -> Maybe (Char, a)) -> a -> Text Source

O(n) Like unfoldr, unfoldrN builds a Text from a seed value. However, the length of the result should be limited by the first argument to unfoldrN. This function is more efficient than unfoldr when the maximum length of the result is known and correct, otherwise its performance is similar to unfoldr. Subject to fusion. Performs replacement on invalid scalar values.

take :: Int64 -> Text -> Text Source

O(n) take n, applied to a Text, returns the prefix of the Text of length n, or the Text itself if n is greater than the length of the Text. Subject to fusion.

takeEnd :: Int64 -> Text -> Text Source

O(n) takeEnd n t returns the suffix remaining after taking n characters from the end of t.

Examples:

takeEnd 3 "foobar" == "bar"

Since: text-1.1.1.0

drop :: Int64 -> Text -> Text Source

O(n) drop n, applied to a Text, returns the suffix of the Text after the first n characters, or the empty Text if n is greater than the length of the Text. Subject to fusion.

dropEnd :: Int64 -> Text -> Text Source

O(n) dropEnd n t returns the prefix remaining after dropping n characters from the end of t.

Examples:

dropEnd 3 "foobar" == "foo"

Since: text-1.1.1.0

takeWhile :: (Char -> Bool) -> Text -> Text Source

O(n) takeWhile, applied to a predicate p and a Text, returns the longest prefix (possibly empty) of elements that satisfy p. Subject to fusion.

takeWhileEnd :: (Char -> Bool) -> Text -> Text Source

O(n) takeWhileEnd, applied to a predicate p and a Text, returns the longest suffix (possibly empty) of elements that satisfy p. Examples:

takeWhileEnd (=='o') "foo" == "oo"

Since: text-1.2.2.0

dropWhile :: (Char -> Bool) -> Text -> Text Source

O(n) dropWhile p t returns the suffix remaining after takeWhile p t. Subject to fusion.

dropWhileEnd :: (Char -> Bool) -> Text -> Text Source

O(n) dropWhileEnd p t returns the prefix remaining after dropping characters that satisfy the predicate p from the end of t.

Examples:

dropWhileEnd (=='.') "foo..." == "foo"

dropAround :: (Char -> Bool) -> Text -> Text Source

O(n) dropAround p t returns the substring remaining after dropping characters that satisfy the predicate p from both the beginning and end of t.

strip :: Text -> Text Source

O(n) Remove leading and trailing white space from a string. Equivalent to:

dropAround isSpace

stripStart :: Text -> Text Source

O(n) Remove leading white space from a string. Equivalent to:

dropWhile isSpace

stripEnd :: Text -> Text Source

O(n) Remove trailing white space from a string. Equivalent to:

dropWhileEnd isSpace

splitAt :: Int64 -> Text -> (Text, Text) Source

O(n) splitAt n t returns a pair whose first element is a prefix of t of length n, and whose second is the remainder of the string. It is equivalent to (take n t, drop n t).

span :: (Char -> Bool) -> Text -> (Text, Text) Source

O(n) span, applied to a predicate p and text t, returns a pair whose first element is the longest prefix (possibly empty) of t of elements that satisfy p, and whose second is the remainder of the list.

breakOn :: Text -> Text -> (Text, Text) Source

O(n+m) Find the first instance of needle (which must be non-null) in haystack. The first element of the returned tuple is the prefix of haystack before needle is matched. The second is the remainder of haystack, starting with the match.

Examples:

breakOn "::" "a::b::c" ==> ("a", "::b::c")
breakOn "/" "foobar"   ==> ("foobar", "")

Laws:

append prefix match == haystack
  where (prefix, match) = breakOn needle haystack

If you need to break a string by a substring repeatedly (e.g. you want to break on every instance of a substring), use breakOnAll instead, as it has lower startup overhead.

This function is strict in its first argument, and lazy in its second.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

breakOnEnd :: Text -> Text -> (Text, Text) Source

O(n+m) Similar to breakOn, but searches from the end of the string.

The first element of the returned tuple is the prefix of haystack up to and including the last match of needle. The second is the remainder of haystack, following the match.

breakOnEnd "::" "a::b::c" ==> ("a::b::", "c")

break :: (Char -> Bool) -> Text -> (Text, Text) Source

O(n) break is like span, but the prefix returned is over elements that fail the predicate p.

group :: Text -> [Text] Source

The group function takes a Text and returns a list of Texts such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result contains only equal elements. For example,

group "Mississippi" = ["M","i","ss","i","ss","i","pp","i"]

It is a special case of groupBy, which allows the programmer to supply their own equality test.

groupBy :: (Char -> Char -> Bool) -> Text -> [Text] Source

The groupBy function is the non-overloaded version of group.

inits :: Text -> [Text] Source

O(n) Return all initial segments of the given Text, shortest first.

tails :: Text -> [Text] Source

O(n) Return all final segments of the given Text, longest first.

Splitting functions in this library do not perform character-wise copies to create substrings; they just construct new Texts that are slices of the original.

splitOn Source

O(m+n) Break a Text into pieces separated by the first Text argument (which cannot be an empty string), consuming the delimiter. An empty delimiter is invalid, and will cause an error to be raised.

Examples:

splitOn "\r\n" "a\r\nb\r\nd\r\ne" == ["a","b","d","e"]
splitOn "aaa"  "aaaXaaaXaaaXaaa"  == ["","X","X","X",""]
splitOn "x"    "x"                == ["",""]

and

intercalate s . splitOn s         == id
splitOn (singleton c)             == split (==c)

(Note: the string s to split on above cannot be empty.)

This function is strict in its first argument, and lazy in its second.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

split :: (Char -> Bool) -> Text -> [Text] Source

O(n) Splits a Text into components delimited by separators, where the predicate returns True for a separator element. The resulting components do not contain the separators. Two adjacent separators result in an empty component in the output. eg.

split (=='a') "aabbaca" == ["","","bb","c",""]
split (=='a') []        == [""]

chunksOf :: Int64 -> Text -> [Text] Source

O(n) Splits a Text into components of length k. The last element may be shorter than the other chunks, depending on the length of the input. Examples:

chunksOf 3 "foobarbaz"   == ["foo","bar","baz"]
chunksOf 4 "haskell.org" == ["hask","ell.","org"]

lines :: Text -> [Text] Source

O(n) Breaks a Text up into a list of Texts at newline Chars. The resulting strings do not contain newlines.

words :: Text -> [Text] Source

O(n) Breaks a Text up into a list of words, delimited by Chars representing white space.

unlines :: [Text] -> Text Source

O(n) Joins lines, after appending a terminating newline to each.

unwords :: [Text] -> Text Source

O(n) Joins words using single space characters.

isPrefixOf :: Text -> Text -> Bool Source

O(n) The isPrefixOf function takes two Texts and returns True iff the first is a prefix of the second. Subject to fusion.

isSuffixOf :: Text -> Text -> Bool Source

O(n) The isSuffixOf function takes two Texts and returns True iff the first is a suffix of the second.

isInfixOf :: Text -> Text -> Bool Source

O(n+m) The isInfixOf function takes two Texts and returns True iff the first is contained, wholly and intact, anywhere within the second.

This function is strict in its first argument, and lazy in its second.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

stripPrefix :: Text -> Text -> Maybe Text Source

O(n) Return the suffix of the second string if its prefix matches the entire first string.

Examples:

stripPrefix "foo" "foobar" == Just "bar"
stripPrefix ""    "baz"    == Just "baz"
stripPrefix "foo" "quux"   == Nothing

This is particularly useful with the ViewPatterns extension to GHC, as follows:

{-# LANGUAGE ViewPatterns #-}
import Data.Text.Lazy as T

fnordLength :: Text -> Int
fnordLength (stripPrefix "fnord" -> Just suf) = T.length suf
fnordLength _                                 = -1

stripSuffix :: Text -> Text -> Maybe Text Source

O(n) Return the prefix of the second string if its suffix matches the entire first string.

Examples:

stripSuffix "bar" "foobar" == Just "foo"
stripSuffix ""    "baz"    == Just "baz"
stripSuffix "foo" "quux"   == Nothing

This is particularly useful with the ViewPatterns extension to GHC, as follows:

{-# LANGUAGE ViewPatterns #-}
import Data.Text.Lazy as T

quuxLength :: Text -> Int
quuxLength (stripSuffix "quux" -> Just pre) = T.length pre
quuxLength _                                = -1

commonPrefixes :: Text -> Text -> Maybe (Text, Text, Text) Source

O(n) Find the longest non-empty common prefix of two strings and return it, along with the suffixes of each string at which they no longer match.

If the strings do not have a common prefix or either one is empty, this function returns Nothing.

Examples:

commonPrefixes "foobar" "fooquux" == Just ("foo","bar","quux")
commonPrefixes "veeble" "fetzer"  == Nothing
commonPrefixes "" "baz"           == Nothing

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

O(n) filter, applied to a predicate and a Text, returns a Text containing those characters that satisfy the predicate.

find :: (Char -> Bool) -> Text -> Maybe Char Source

O(n) The find function takes a predicate and a Text, and returns the first element in matching the predicate, or Nothing if there is no such element. Subject to fusion.

breakOnAll Source

O(n+m) Find all non-overlapping instances of needle in haystack. Each element of the returned list consists of a pair:

• The entire string prior to the kth match (i.e. the prefix)
• The kth match, followed by the remainder of the string

Examples:

breakOnAll "::" ""
==> []
breakOnAll "/" "a/b/c/"
==> [("a", "/b/c/"), ("a/b", "/c/"), ("a/b/c", "/")]

This function is strict in its first argument, and lazy in its second.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

The needle parameter may not be empty.

partition :: (Char -> Bool) -> Text -> (Text, Text) Source

O(n) The partition function takes a predicate and a Text, and returns the pair of Texts with elements which do and do not satisfy the predicate, respectively; i.e.

partition p t == (filter p t, filter (not . p) t)

index :: Text -> Int64 -> Char Source

O(n) Text index (subscript) operator, starting from 0. Subject to fusion.

count :: Text -> Text -> Int64 Source

O(n+m) The count function returns the number of times the query string appears in the given Text. An empty query string is invalid, and will cause an error to be raised.

In (unlikely) bad cases, this function's time complexity degrades towards O(n*m).

zip :: Text -> Text -> [(Char, Char)] Source

O(n) zip takes two Texts and returns a list of corresponding pairs of bytes. If one input Text is short, excess elements of the longer Text are discarded. This is equivalent to a pair of unpack operations.

zipWith :: (Char -> Char -> Char) -> Text -> Text -> Text Source

O(n) zipWith generalises zip by zipping with the function given as the first argument, instead of a tupling function. Performs replacement on invalid scalar values.

Orphan instances