Data.Text.Lazy

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.

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

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

O(n) Concatenate a list of Texts.

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

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.

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"

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

O(n²) Return all initial segments of the given Text, shortest first.

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.

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

O(n+m) The isInfixOf function takes two Texts and returns True if and only if 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).

O(n) The isPrefixOf function takes two Texts and returns True if and only if the first is a prefix of the second.

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

O(n) Breaks a Text up into a list of Texts at newline characters '\n' (LF, line feed). The resulting strings do not contain newlines.

lines does not treat '\r' (CR, carriage return) as a newline character.

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)

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

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

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

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.

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.

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

O(n) Joins words using single space characters.

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

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.

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.

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

>>> T.break (=='c') "180cm"("180","cm")

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

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

O(n) A strict version of foldl1.

O(1) Returns the first character of a Text, which must be non-empty. This is a partial function, consider using uncons instead.

O(n/c) Returns all but the last character of a Text, which must be non-empty. This is a partial function, consider using unsnoc instead.

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

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

O(n/c) Returns the last character of a Text, which must be non-empty. This is a partial function, consider using unsnoc instead.

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

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

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

O(n) scanl is similar to foldl, but returns a list of successive reduced values from the left. 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.

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, ...]

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

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

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 text.

>>> T.span (=='0') "000AB"("000","AB")

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).

O(1) Returns all characters after the head of a Text, which must be non-empty. This is a partial function, consider using uncons instead.

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.

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

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

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

• It is no faster than using init and last.

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.

Write a string to the end of a file.

Lazily read all user input on stdin as a single string.

Read a single line of user input from stdin.

The interact function takes a function of type Text -> Text as its argument. The entire input from the standard input device is passed (lazily) to this function as its argument, and the resulting string is output on the standard output device.

Write a string to stdout.

Write a string to stdout, followed by a newline.

Read a file and return its contents as a string. The file is read lazily, as with getContents.

Beware that this function (similarly to readFile) is locale-dependent. Unexpected system locale may cause your application to read corrupted data or throw runtime exceptions about "invalid argument (invalid byte sequence)" or "invalid argument (invalid character)". This is also slow, because GHC first converts an entire input to UTF-32, which is afterwards converted to UTF-8.

If your data is UTF-8, using decodeUtf8 . readFile is a much faster and safer alternative.

Write a string to a file. The file is truncated to zero length before writing begins.

O(n) Test whether Text contains only ASCII code-points (i.e. only U+0000 through U+007F).

This is a more efficient version of all isAscii.

>>> isAscii ""True

>>> isAscii "abc\NUL"True

>>> isAscii "abcd€"False

prop> isAscii t == all (< '\x80') t

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).

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

The first letter (as determined by isLetter) 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).

This function is not idempotent. Consider lower-case letter ʼn (U+0149 LATIN SMALL LETTER N PRECEDED BY APOSTROPHE). Then toTitle "ʼn" = "ʼN": the first (and the only) letter of the input is converted to title case, becoming two letters. Now ʼ (U+02BC MODIFIER LETTER APOSTROPHE) is a modifier letter and as such is recognised as a letter by isLetter, so toTitle "ʼN" = "'n".

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.

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.

Smart constructor for Empty.

O(n) The elem function takes a character and a Text, and returns True if the element is found in the given Text, or False otherwise.

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.

O(n) A strict version of foldl.

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

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.

foldr is lazy like foldr for lists: evaluation actually traverses the Text from left to right, only as far as it needs to.

For example, head can be defined with O(1) complexity using foldr:

head :: Text -> Char
head = foldr const (error "head empty")

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

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

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

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

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

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

Convert a value to lazy Text.

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

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.

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

O(n) Convert a Text into a String.

Decode a ByteString containing UTF-8 encoded text..

If the input contains any invalid UTF-8 data, the relevant exception will be returned, otherwise the decoded text.

Note: this function is not lazy, as it must decode its entire input before it can return a result. If you need lazy (streaming) decoding, use decodeUtf8With in lenient mode.

Encode text to a ByteString Builder using UTF-8 encoding.

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

O(n) Convert a String into a Text.

Performs replacement on invalid scalar values, so unpack . pack is not id:

>>> Data.Text.Lazy.unpack (Data.Text.Lazy.pack "\55555")"\65533"

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.

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

O(n) A monadic version of foldl'.

O(n) Adds a character to the end of a Text. This copies the entire array in the process.

Bidirectional pattern synonym for cons (O(n)) and uncons (O(1)), to be used together with Empty.

Bidirectional pattern synonym for snoc (O(n)) and unsnoc (O(1)) to be used together with Empty.

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).

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).

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).

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.

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")

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"

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"]

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

O(min(n,c)) Compare the count of characters in a Text to a number.

compareLength t c = compare (length t) c

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.

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.

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

Examples:

dropEnd 3 "foobar" == "foo"

O(n²) Return all initial segments of the given Text, shortest first.

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

Examples:

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

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"

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).

O(length of suffix) spanEndM, applied to a monadic predicate p, a text t, returns a pair (t1, t2) where t2 is the longest suffix of t whose elements satisfy p, and t1 is the remainder of the text.

>>> T.spanEndM (\c -> state $ \i -> (fromEnum c == i, i-1)) "tuvxyz" `runState` 122(("tuv","xyz"),118)

spanEndM p . reverse = fmap (bimap reverse reverse) . spanM p

O(length of prefix) spanM, applied to a monadic predicate p, a text t, returns a pair (t1, t2) where t1 is the longest prefix of t whose elements satisfy p, and t2 is the remainder of the text.

>>> T.spanM (\c -> state $ \i -> (fromEnum c == i, i+1)) "abcefg" `runState` 97(("abc","efg"),101)

span is spanM specialized to Identity:

-- for all p :: Char -> Bool
span p = runIdentity . spanM (pure . p)

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') []        == [""]

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

dropAround isSpace

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

dropWhileEnd isSpace

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

dropWhile isSpace

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

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

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

Examples:

takeEnd 3 "foobar" == "bar"

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"

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.

Type synonym for the lazy flavour of Text.

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

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

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

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

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

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

Decode a ByteString containing UTF-8 encoded text.

Lazily read the remaining contents of a Handle. The Handle will be closed after the read completes, or on error.

Read a single line from a handle.

Write a string to a handle.

Write a string to a handle, followed by a newline.

Encode text using UTF-8 encoding.

Decode a ByteString containing 7-bit ASCII encoded text.

Decode a ByteString containing Latin-1 (aka ISO-8859-1) encoded text.

Decode text from big endian UTF-16 encoding.

If the input contains any invalid big endian UTF-16 data, an exception will be thrown. For more control over the handling of invalid data, use decodeUtf16BEWith.

Decode text from big endian UTF-16 encoding.

Decode text from little endian UTF-16 encoding.

If the input contains any invalid little endian UTF-16 data, an exception will be thrown. For more control over the handling of invalid data, use decodeUtf16LEWith.

Decode text from little endian UTF-16 encoding.

Decode text from big endian UTF-32 encoding.

If the input contains any invalid big endian UTF-32 data, an exception will be thrown. For more control over the handling of invalid data, use decodeUtf32BEWith.

Decode text from big endian UTF-32 encoding.

Decode text from little endian UTF-32 encoding.

If the input contains any invalid little endian UTF-32 data, an exception will be thrown. For more control over the handling of invalid data, use decodeUtf32LEWith.

Decode text from little endian UTF-32 encoding.

Decode a ByteString containing UTF-8 encoded text that is known to be valid.

If the input contains any invalid UTF-8 data, an exception will be thrown that cannot be caught in pure code. For more control over the handling of invalid data, use decodeUtf8' or decodeUtf8With.

Encode text using big endian UTF-16 encoding.

Encode text using little endian UTF-16 encoding.

Encode text using big endian UTF-32 encoding.

Encode text using little endian UTF-32 encoding.

Encode text using UTF-8 encoding and escape the ASCII characters using a BoundedPrim.

Use this function is to implement efficient encoders for text-based formats like JSON or HTML.