:: Eq a => [a] -> [[a]] -package:ghc

The group function takes a list and returns a list of lists such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result is non-empty, all elements are equal to the first one, and consecutive equal elements of the input end up in the same element of the output list. group is a special case of groupBy, which allows the programmer to supply their own equality test. It's often preferable to use Data.List.NonEmpty.group, which provides type-level guarantees of non-emptiness of inner lists. A common idiom to squash repeating elements map head . group is better served by map Data.List.NonEmpty.head . Data.List.NonEmpty.group because it avoids partial functions.

Examples

>>> group "Mississippi"
["M","i","ss","i","ss","i","pp","i"]

>>> group [1, 1, 1, 2, 2, 3, 4, 5, 5]
[[1,1,1],[2,2],[3],[4],[5,5]]

The group function takes a list and returns a list of lists such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result is non-empty and all elements are equal to the first one. group is a special case of groupBy, which allows the programmer to supply their own equality test. It's often preferable to use Data.List.NonEmpty.group, which provides type-level guarantees of non-emptiness of inner lists.

Examples

>>> group "Mississippi"
["M","i","ss","i","ss","i","pp","i"]

>>> group [1, 1, 1, 2, 2, 3, 4, 5, 5]
[[1,1,1],[2,2],[3],[4],[5,5]]

The group function takes a list and returns a list of lists 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.

Classify values using their Eq instance.

> classify [1,2,3,1,2,1]
[[1,1,1],[2,2],[3]]

(cf. classifyBy, classifyOn)

The group function takes a list and returns a list of lists such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result is non-empty and all elements are equal to the first one. For example,

>>> group "Mississippi"
["M","i","ss","i","ss","i","pp","i"]

group is a special case of groupBy, which allows the programmer to supply their own equality test. It's often preferable to use Data.List.NonEmpty.group, which provides type-level guarantees of non-emptiness of inner lists.

The subsequences function returns the list of all subsequences of the argument.

Laziness

subsequences does not look ahead unless it must:

>>> take 1 (subsequences undefined)
[[]]

>>> take 2 (subsequences ('a' : undefined))
["","a"]

Examples

>>> subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

This function is productive on infinite inputs:

>>> take 8 $ subsequences ['a'..]
["","a","b","ab","c","ac","bc","abc"]

The permutations function returns the list of all permutations of the argument. Note that the order of permutations is not lexicographic. It satisfies the following property:

map (take n) (take (product [1..n]) (permutations ([1..n] ++ undefined))) == permutations [1..n]

Laziness

The permutations function is maximally lazy: for each n, the value of permutations xs starts with those permutations that permute take n xs and keep drop n xs.

Examples

>>> permutations "abc"
["abc","bac","cba","bca","cab","acb"]

>>> permutations [1, 2]
[[1,2],[2,1]]

>>> permutations []
[[]]

This function is productive on infinite inputs:

>>> take 6 $ map (take 3) $ permutations ['a'..]
["abc","bac","cba","bca","cab","acb"]

The inits function returns all initial segments of the argument, shortest first. inits is semantically equivalent to map reverse . scanl (flip (:)) [], but under the hood uses a queue to amortize costs of reverse.

Laziness

Note that inits has the following strictness property: inits (xs ++ _|_) = inits xs ++ _|_ In particular, inits _|_ = [] : _|_

Examples

>>> inits "abc"
["","a","ab","abc"]

>>> inits []
[[]]

inits is productive on infinite lists:

>>> take 5 $ inits [1..]
[[],[1],[1,2],[1,2,3],[1,2,3,4]]

The tails function returns all final segments of the argument, longest first.

Laziness

Note that tails has the following strictness property: tails _|_ = _|_ : _|_

>>> tails undefined
[*** Exception: Prelude.undefined

>>> drop 1 (tails [undefined, 1, 2])
[[1, 2], [2], []]

Examples

>>> tails "abc"
["abc","bc","c",""]

>>> tails [1, 2, 3]
[[1,2,3],[2,3],[3],[]]

>>> tails []
[[]]

Shrink a list by edging towards the empty list.

>>> list [1,2,3]
[[],[2,3],[1,3],[1,2]]

>>> list "abcd"
["","cd","ab","bcd","acd","abd","abc"]

Note we always try the empty list first, as that is the optimal shrink.

This function is lazier than the one suggested in the Haskell 98 report. It is inits undefined = [] : undefined, in contrast to Data.List.inits undefined = undefined.

This function is lazier than the one suggested in the Haskell 98 report. It is tails undefined = ([] : undefined) : undefined, in contrast to Data.List.tails undefined = undefined.

The subsequences function returns the list of all subsequences of the argument.

>>> subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

The permutations function returns the list of all permutations of the argument.

>>> permutations "abc"
["abc","bac","cba","bca","cab","acb"]

The inits function returns all initial segments of the argument, shortest first. For example,

>>> inits "abc"
["","a","ab","abc"]

Note that inits has the following strictness property: inits (xs ++ _|_) = inits xs ++ _|_ In particular, inits _|_ = [] : _|_

The tails function returns all final segments of the argument, longest first. For example,

>>> tails "abc"
["abc","bc","c",""]

Note that tails has the following strictness property: tails _|_ = _|_ : _|_

List.inits is defined by inits = foldr (x ys -> [] : map (x:) ys) [[]] This is too strict for our application.

Prelude> List.inits (0:1:2:undefined)
[[],[0],[0,1]*** Exception: Prelude.undefined

The following routine is more lazy than inits and even lazier than inits from utility-ht package, but it is restricted to infinite lists. This degree of laziness is needed for sqrtFP.

Prelude> lazyInits (0:1:2:undefined)
[[],[0],[0,1],[0,1,2],[0,1,2,*** Exception: Prelude.undefined

The inits function returns all initial segments of the argument, shortest first. For example,

>>> inits "abc"
["","a","ab","abc"]

Note that inits has the following strictness property: inits (xs ++ _|_) = inits xs ++ _|_ In particular, inits _|_ = [] : _|_ inits is semantically equivalent to map reverse . scanl (flip (:)) [], but under the hood uses a queue to amortize costs of reverse.

The subsequences function returns the list of all subsequences of the argument.

>>> subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

This function is productive on infinite inputs:

>>> take 8 $ subsequences ['a'..]
["","a","b","ab","c","ac","bc","abc"]

The permutations function returns the list of all permutations of the argument.

>>> permutations "abc"
["abc","bac","cba","bca","cab","acb"]

The permutations function is maximally lazy: for each n, the value of permutations xs starts with those permutations that permute take n xs and keep drop n xs. This function is productive on infinite inputs:

>>> take 6 $ map (take 3) $ permutations ['a'..]
["abc","bac","cba","bca","cab","acb"]

Note that the order of permutations is not lexicographic. It satisfies the following property:

map (take n) (take (product [1..n]) (permutations ([1..n] ++ undefined))) == permutations [1..n]

Takes a list of values xs and transform it into tiers on which each tier is occupied by a single element from xs.

toTiers [x, y, z, ...]  =  [[x], [y], [z], ...]

To convert back to a list, just concat.

Split a list into lists that are small enough to have a corresponding tuple arity. The sub-lists of the result all have length <= mAX_TUPLE_SIZE But there may be more than mAX_TUPLE_SIZE sub-lists

Chunk in groups of 4. (This function must be in some standard library, where?)

Return all combinations of a sequence of values.