Data.Text.Lazy

A time and space-efficient implementation of Unicode text using lists of packed arrays. This representation 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 complexity than their Data.Text equivalents, due to optimisations resulting from the list spine structure. And for other operations lazy Texts are usually within a few percent of strict ones, but with better heap usage. 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 B

Documentation

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.

This function is subject to array fusion.

unpack :: Text -> String

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

singleton :: Char -> Text

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.

Basic interface

cons :: Char -> Text -> Text

snoc :: Text -> Char -> Text

append :: Text -> Text -> Text

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

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

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

head :: Text -> Char

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

last :: Text -> Char

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

tail :: Text -> Text

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

init :: Text -> Text

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

null :: Text -> Bool

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

length :: Text -> Int64

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 array fusion.

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 array fusion.

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.

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

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. 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. 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. Examples:

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

Folds

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

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 array fusion.

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

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

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

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

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

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 array fusion.

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

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

maximum :: Text -> Char

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

minimum :: Text -> Char

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

 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. This function is subject to array fusion.

 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.

 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.

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.

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

The mapAccumR function behaves like a combination of map and 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.

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.

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.

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.

drop :: Int64 -> Text -> Text

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

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. This function is subject to array fusion.

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

O(n) dropWhile p t returns the suffix remaining after takeWhile p t. This function is subject to array 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).

spanBy :: (Char -> Bool) -> Text -> (Text, Text)

O(n) spanBy, 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.

break :: 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:

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

Laws:

 append prefix match == haystack
   where (prefix, match) = break 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 find 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).

breakEnd :: Text -> Text -> (Text, Text)

O(n+m) Similar to break, 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.

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

breakBy :: (Char -> Bool) -> Text -> (Text, Text)

O(n) breakBy is like spanBy, 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.

split

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:

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

and

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

splitBy :: (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.

 splitBy (=='a') "aabbaca" == ["","","bb","c",""]
 splitBy (=='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. This function is 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).

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

O(n+m) Find all non-overlapping instances of needle in haystack. The first element of the returned pair is the prefix of haystack prior to any matches of needle. The second is a list of pairs.

The first element of each pair in the list is a span from the beginning of a match to the beginning of the next match, while the second is a span from the beginning of the match to the end of the input.

Examples:

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

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).

findBy :: (Char -> Bool) -> Text -> Maybe Char

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

partitionBy :: (Char -> Bool) -> Text -> (Text, Text)

O(n) The partitionBy 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.

 partitionBy 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.