Data.Text.Lazy

A time and space-efficient implementation of Unicode text using lists of packed arrays.

Note: Read below the synopsis for important notes on the use of this module.

The representation used by this module is suitable for high performance use and for streaming large quantities of data. It provides a means to manipulate a large body of text without requiring that the entire content be resident in memory.

Some operations, such as concat, append, reverse and cons, have better time complexity than their Data.Text equivalents, due to the underlying representation being a list of chunks. For other operations, lazy Texts are usually within a few percent of strict ones, but often with better heap usage if used in a streaming fashion. For data larger than available memory, or if you have tight memory constraints, this module will be the only option.

This module is intended to be imported qualified, to avoid name clashes with Prelude functions. eg.

 import qualified Data.Text.Lazy as L

Fusion

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".

Acceptable data

A Text value is a sequence of Unicode scalar values, as defined in §3.9, definition D76 of the Unicode 5.2 standard: http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf#page=35. 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: http://unicode.org/reports/tr36#Deletion_of_Noncharacters)

Types

data Text

Instances

Eq Text

Data Text

Ord Text

Read Text

Show Text

Typeable Text

IsString Text

Monoid Text

NFData Text

Creation and elimination

pack :: String -> Text

O(n) Convert a String into a Text.

Subject to fusion. Performs replacement on invalid scalar values.

unpack :: Text -> String

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

singleton :: Char -> Text

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

empty :: Text

Smart constructor for Empty.

fromChunks :: [Text] -> Text

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

toChunks :: Text -> [Text]

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

toStrict :: Text -> Text

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

fromStrict :: Text -> Text

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

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

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

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

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

Basic interface

cons :: Char -> Text -> Text

O(n) Adds a character to the front of a Text. This function is more costly than its List counterpart because it requires copying a new array. Subject to fusion.

snoc :: Text -> Char -> Text

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

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

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

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

head :: Text -> Char

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

last :: Text -> Char

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

tail :: Text -> Text

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

init :: Text -> Text

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

null :: Text -> Bool

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

length :: Text -> Int64

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

compareLength :: Text -> Int64 -> Ordering

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.

Transformations

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

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

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

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]

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

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

replace

Arguments

:: Text

Text to search for

-> Text

Replacement text

-> Text

Input text

-> Text

O(m+n) Replace every occurrence of one substring with another.

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

Case conversion

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

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

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

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

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.

Justification

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

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

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

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"

Folds

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

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

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

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

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

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

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

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

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.

Special folds

concat :: [Text] -> Text

O(n) Concatenate a list of Texts.

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

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

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

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

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

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

maximum :: Text -> Char

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

minimum :: Text -> Char

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

Construction

Scans

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

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

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

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

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

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

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

Accumulating maps

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

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)

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.

Generation and unfolding

replicate :: Int64 -> Text -> Text

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

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

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. Performs replacement on invalid scalar values.

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

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. Performs replacement on invalid scalar values.

Substrings

Breaking strings

take :: Int64 -> Text -> Text

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

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

Examples:

 takeEnd 3 "foobar" == "bar"

drop :: Int64 -> Text -> Text

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

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

Examples:

 dropEnd 3 "foobar" == "foo"

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

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.

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

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

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

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

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

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

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

strip :: Text -> Text

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

 dropAround isSpace

stripStart :: Text -> Text

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

 dropWhile isSpace

stripEnd :: Text -> Text

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

 dropWhileEnd isSpace

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

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)

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)

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)

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)

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

group :: Text -> [Text]

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]

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

inits :: Text -> [Text]

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

tails :: Text -> [Text]

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

Breaking into many substrings

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

Arguments

:: Text

Text to split on

-> Text

Input text

-> [Text]

O(m+n) Break a Text into pieces separated by the first Text argument, 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)

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]

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]

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"]

Breaking into lines and words

lines :: Text -> [Text]

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

words :: Text -> [Text]

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

unlines :: [Text] -> Text

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

unwords :: [Text] -> Text

O(n) Joins words using single space characters.

Predicates

isPrefixOf :: Text -> Text -> Bool

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

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

isInfixOf :: Text -> Text -> Bool

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).

View patterns

stripPrefix :: Text -> Text -> Maybe Text

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

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)

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

Searching

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

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

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.

breakOnAll

Arguments

:: Text

needle to search for

-> Text

haystack in which to search

-> [(Text, Text)]

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)

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)

Indexing

index :: Text -> Int64 -> Char

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

count :: Text -> Text -> Int64

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).

Zipping and unzipping

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

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

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.