Data.Text.Lazy

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.

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 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.

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) 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(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) Concatenate a list of Texts.

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

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

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

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(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) 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) 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.

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.

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

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

Examples:

takeEnd 3 "foobar" == "bar"

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

Examples:

dropEnd 3 "foobar" == "foo"

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

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) dropWhile p t returns the suffix remaining after takeWhile p t.

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"

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

dropAround isSpace

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

dropWhile isSpace

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

dropWhileEnd isSpace

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

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

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.

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

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

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

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

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

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

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

O(n) Joins words using single space characters.

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 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) 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