Data.Text.Lazy

text-1.2.2.1: An efficient packed Unicode text type.

Copyright | (c) 2009, 2010, 2012 Bryan O'Sullivan
License | BSD-style
Maintainer | bos@serpentine.com
Stability | experimental
Portability | GHC
Safe Haskell | Trustworthy
Language | Haskell98

Data.Text.Lazy

Contents

- Fusion
- Acceptable data
- Types
- Creation and elimination
- Basic interface
- Transformations
  - Case conversion
  - Justification
- Folds
  - Special folds
- Construction
  - Scans
  - Accumulating maps
  - Generation and unfolding
- Substrings
  - Breaking strings
  - Breaking into many substrings
  - Breaking into lines and words
- Predicates
  - View patterns
- Searching
- Indexing
- Zipping and unzipping

Description

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 `Text`s 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

Synopsis

- data Text
- pack :: String -> Text
- unpack :: Text -> String
- singleton :: Char -> Text
- empty :: Text
- fromChunks :: [Text] -> Text
- toChunks :: Text -> [Text]
- toStrict :: Text -> Text
- fromStrict :: Text -> Text
- foldrChunks :: (Text -> a -> a) -> a -> Text -> a
- foldlChunks :: (a -> Text -> a) -> a -> Text -> a
- cons :: Char -> Text -> Text
- snoc :: Text -> Char -> Text
- append :: Text -> Text -> Text
- uncons :: Text -> Maybe (Char, Text)
- head :: Text -> Char
- last :: Text -> Char
- tail :: Text -> Text
- init :: Text -> Text
- null :: Text -> Bool
- length :: Text -> Int64
- compareLength :: Text -> Int64 -> Ordering
- map :: (Char -> Char) -> Text -> Text
- intercalate :: Text -> [Text] -> Text
- intersperse :: Char -> Text -> Text
- transpose :: [Text] -> [Text]
- reverse :: Text -> Text
- replace :: Text -> Text -> Text -> Text
- toCaseFold :: Text -> Text
- toLower :: Text -> Text
- toUpper :: Text -> Text
- toTitle :: Text -> Text
- justifyLeft :: Int64 -> Char -> Text -> Text
- justifyRight :: Int64 -> Char -> Text -> Text
- center :: Int64 -> Char -> Text -> Text
- foldl :: (a -> Char -> a) -> a -> Text -> a
- foldl' :: (a -> Char -> a) -> a -> Text -> a
- foldl1 :: (Char -> Char -> Char) -> Text -> Char
- foldl1' :: (Char -> Char -> Char) -> Text -> Char
- foldr :: (Char -> a -> a) -> a -> Text -> a
- foldr1 :: (Char -> Char -> Char) -> Text -> Char
- concat :: [Text] -> Text
- concatMap :: (Char -> Text) -> Text -> Text
- any :: (Char -> Bool) -> Text -> Bool
- all :: (Char -> Bool) -> Text -> Bool
- maximum :: Text -> Char
- minimum :: Text -> Char
- scanl :: (Char -> Char -> Char) -> Char -> Text -> Text
- scanl1 :: (Char -> Char -> Char) -> Text -> Text
- scanr :: (Char -> Char -> Char) -> Char -> Text -> Text
- scanr1 :: (Char -> Char -> Char) -> Text -> Text
- mapAccumL :: (a -> Char -> (a, Char)) -> a -> Text -> (a, Text)
- mapAccumR :: (a -> Char -> (a, Char)) -> a -> Text -> (a, Text)
- repeat :: Char -> Text
- replicate :: Int64 -> Text -> Text
- cycle :: Text -> Text
- iterate :: (Char -> Char) -> Char -> Text
- unfoldr :: (a -> Maybe (Char, a)) -> a -> Text
- unfoldrN :: Int64 -> (a -> Maybe (Char, a)) -> a -> Text
- take :: Int64 -> Text -> Text
- takeEnd :: Int64 -> Text -> Text
- drop :: Int64 -> Text -> Text
- dropEnd :: Int64 -> Text -> Text
- takeWhile :: (Char -> Bool) -> Text -> Text
- takeWhileEnd :: (Char -> Bool) -> Text -> Text
- dropWhile :: (Char -> Bool) -> Text -> Text
- dropWhileEnd :: (Char -> Bool) -> Text -> Text
- dropAround :: (Char -> Bool) -> Text -> Text
- strip :: Text -> Text
- stripStart :: Text -> Text
- stripEnd :: Text -> Text
- splitAt :: Int64 -> Text -> (Text, Text)
- span :: (Char -> Bool) -> Text -> (Text, Text)
- breakOn :: Text -> Text -> (Text, Text)
- breakOnEnd :: Text -> Text -> (Text, Text)
- break :: (Char -> Bool) -> Text -> (Text, Text)
- group :: Text -> [Text]
- groupBy :: (Char -> Char -> Bool) -> Text -> [Text]
- inits :: Text -> [Text]
- tails :: Text -> [Text]
- splitOn :: Text -> Text -> [Text]
- split :: (Char -> Bool) -> Text -> [Text]
- chunksOf :: Int64 -> Text -> [Text]
- lines :: Text -> [Text]
- words :: Text -> [Text]
- unlines :: [Text] -> Text
- unwords :: [Text] -> Text
- isPrefixOf :: Text -> Text -> Bool
- isSuffixOf :: Text -> Text -> Bool
- isInfixOf :: Text -> Text -> Bool
- stripPrefix :: Text -> Text -> Maybe Text
- stripSuffix :: Text -> Text -> Maybe Text
- commonPrefixes :: Text -> Text -> Maybe (Text, Text, Text)
- filter :: (Char -> Bool) -> Text -> Text
- find :: (Char -> Bool) -> Text -> Maybe Char
- breakOnAll :: Text -> Text -> [(Text, Text)]
- partition :: (Char -> Bool) -> Text -> (Text, Text)
- index :: Text -> Int64 -> Char
- count :: Text -> Text -> Int64
- zip :: Text -> Text -> [(Char, Char)]
- zipWith :: (Char -> Char -> Char) -> Text -> Text -> Text

# 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

type Item Text = Char |

# 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 `Text`s into a lazy `Text`.

toChunks :: Text -> [Text]

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

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 infixr 5

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 `Text`s 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 | `needle` to search for. If this string is empty, an error will occur.
-> Text | `replacement` to replace `needle` with.
-> Text | `haystack` in which to search.
-> Text |

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).

## 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 `Text`s.

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` satisfies the predicate `p`. Subject to fusion.

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

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

repeat :: Char -> Text

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

replicate :: Int64 -> Text -> Text

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

cycle :: Text -> Text

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

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

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

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

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.

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

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"

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 satisfy 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 satisfy 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 `Text`s 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 `Text`s that are slices of the original.

splitOn

Arguments

:: Text | String to split on. If this string is empty, an error will occur.
-> Text | Input text.
-> [Text] |

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]

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 `Text`s at newline `Char`s. The resulting strings do not contain newlines.

words :: Text -> [Text]

O(n) Breaks a `Text` up into a list of words, delimited by `Char`s 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 `Text`s 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 `Text`s and returns `True` iff the first is a suffix of the second.

isInfixOf :: Text -> Text -> Bool

O(n+m) The `isInfixOf` function takes two `Text`s 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 `Text`s 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 `Text`s 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.