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List operations.
Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space, while a non-tail-recursive function uses stack space proportional to the length of its list argument, which can be a problem with very long lists. When the function takes several list arguments, an approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.
The above considerations can usually be ignored if your lists are not longer than about 10000 elements.
The labeled version of this module can be used as described in the StdLabels module.
type'a t = 'a list =
| []
| ::of'a * 'a list
An alias for the type of lists.
val length : 'a list-> int
Return the length (number of elements) of the given list.
val compare_lengths : 'a list->'b list-> int
Compare the lengths of two lists. compare_lengths l1 l2 is equivalent to compare (length l1) (length l2), except that the computation stops after reaching the end of the shortest list.
since 4.05
val compare_length_with : 'a list->len:int -> int
Compare the length of a list to an integer. compare_length_with l len is equivalent to compare (length l) len, except that the computation stops after at most len iterations on the list.
since 4.05
val is_empty : 'a list-> bool
is_empty l is true if and only if l has no elements. It is equivalent to compare_length_with l 0 = 0.
append l0 l1 appends l1 to l0. Same function as the infix operator @.
since 5.1 this function is tail-recursive.
val rev_append : 'a list->'a list->'a list
rev_append l1 l2 reverses l1 and concatenates it with l2. This is equivalent to (rev l1) @ l2.
val concat : 'a list list->'a list
Concatenate a list of lists. The elements of the argument are all concatenated together (in the same order) to give the result. Not tail-recursive (length of the argument + length of the longest sub-list).
val flatten : 'a list list->'a list
Same as concat. Not tail-recursive (length of the argument + length of the longest sub-list).
Comparison
val equal : eq:('a->'a-> bool)->'a list->'a list-> bool
equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have the same length, and for each pair of elements ai, bi at the same position we have eq ai bi.
Note: the eq function may be called even if the lists have different length. If you know your equality function is costly, you may want to check compare_lengths first.
since 4.12
val compare : cmp:('a->'a-> int)->'a list->'a list-> int
compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic comparison of the two input lists, using the same 'a -> 'a -> int interface as Stdlib.compare:
a1 :: l1 is smaller than a2 :: l2 (negative result) if a1 is smaller than a2, or if they are equal (0 result) and l1 is smaller than l2
the empty list [] is strictly smaller than non-empty lists
Note: the cmp function will be called even if the lists have different lengths.
since 4.12
Iterators
val iter : f:('a-> unit)->'a list-> unit
iter ~f [a1; ...; an] applies function f in turn to [a1; ...; an]. It is equivalent to f a1; f a2; ...; f an.
val iteri : f:(int ->'a-> unit)->'a list-> unit
Same as iter, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.
since 4.00
val map : f:('a->'b)->'a list->'b list
map ~f [a1; ...; an] applies function f to a1, ..., an, and builds the list [f a1; ...; f an] with the results returned by f.
val mapi : f:(int ->'a->'b)->'a list->'b list
Same as map, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.
since 4.00
val rev_map : f:('a->'b)->'a list->'b list
rev_map ~f l gives the same result as rev (map f l), but is more efficient.
val filter_map : f:('a->'b option)->'a list->'b list
filter_map ~f l applies f to every element of l, filters out the None elements and returns the list of the arguments of the Some elements.
since 4.08
val concat_map : f:('a->'b list)->'a list->'b list
concat_map ~f l gives the same result as concat (map f l). Tail-recursive.
since 4.10
val fold_left_map :
f:('acc->'a->'acc * 'b)->init:'acc->'a list->'acc * 'b list
fold_left_map is a combination of fold_left and map that threads an accumulator through calls to f.
since 4.11
val fold_left : f:('acc->'a->'acc)->init:'acc->'a list->'acc
fold_left ~f ~init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn.
val fold_right : f:('a->'acc->'acc)->'a list->init:'acc->'acc
fold_right ~f [a1; ...; an] ~init is f a1 (f a2 (... (f an init) ...)). Not tail-recursive.
Iterators on two lists
val iter2 : f:('a->'b-> unit)->'a list->'b list-> unit
iter2 ~f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn.
if the two lists are determined to have different lengths. Not tail-recursive.
List scanning
val for_all : f:('a-> bool)->'a list-> bool
for_all ~f [a1; ...; an] checks if all elements of the list satisfy the predicate f. That is, it returns (f a1) && (f a2) && ... && (f an) for a non-empty list and true if the list is empty.
val exists : f:('a-> bool)->'a list-> bool
exists ~f [a1; ...; an] checks if at least one element of the list satisfies the predicate f. That is, it returns (f a1) || (f a2) || ... || (f an) for a non-empty list and false if the list is empty.
val for_all2 : f:('a->'b-> bool)->'a list->'b list-> bool
Same as for_all, but for a two-argument predicate.
if there is no value that satisfies f in the list l.
val find_opt : f:('a-> bool)->'a list->'a option
find ~f l returns the first element of the list l that satisfies the predicate f. Returns None if there is no value that satisfies f in the list l.
since 4.05
val find_index : f:('a-> bool)->'a list->int option
find_index ~f xs returns Some i, where i is the index of the first element of the list xs that satisfies f x, if there is such an element.
It returns None if there is no such element.
since 5.1
val find_map : f:('a->'b option)->'a list->'b option
find_map ~f l applies f to the elements of l in order, and returns the first result of the form Some v, or None if none exist.
since 4.10
val find_mapi : f:(int ->'a->'b option)->'a list->'b option
Same as find_map, but the predicate is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.
since 5.1
val filter : f:('a-> bool)->'a list->'a list
filter ~f l returns all the elements of the list l that satisfy the predicate f. The order of the elements in the input list is preserved.
val filteri : f:(int ->'a-> bool)->'a list->'a list
Same as filter, but the predicate is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.
since 4.11
val partition : f:('a-> bool)->'a list->'a list * 'a list
partition ~f l returns a pair of lists (l1, l2), where l1 is the list of all the elements of l that satisfy the predicate f, and l2 is the list of all the elements of l that do not satisfy f. The order of the elements in the input list is preserved.
val partition_map : f:('a->('b, 'c)Either.t)->'a list->'b list * 'c list
partition_map f l returns a pair of lists (l1, l2) such that, for each element x of the input list l:
if f x is Left y1, then y1 is in l1, and
if f x is Right y2, then y2 is in l2.
The output elements are included in l1 and l2 in the same relative order as the corresponding input elements in l.
In particular, partition_map (fun x -> if f x then Left x else Right x) l is equivalent to partition f l.
since 4.12
Association lists
val assoc : 'a->('a * 'b) list->'b
assoc a l returns the value associated with key a in the list of pairs l. That is, assoc a [ ...; (a,b); ...] = b if (a,b) is the leftmost binding of a in list l.
if there is no value associated with a in the list l.
val assoc_opt : 'a->('a * 'b) list->'b option
assoc_opt a l returns the value associated with key a in the list of pairs l. That is, assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l. Returns None if there is no value associated with a in the list l.
since 4.05
val assq : 'a->('a * 'b) list->'b
Same as assoc, but uses physical equality instead of structural equality to compare keys.
val assq_opt : 'a->('a * 'b) list->'b option
Same as assoc_opt, but uses physical equality instead of structural equality to compare keys.
since 4.05
val mem_assoc : 'a->map:('a * 'b) list-> bool
Same as assoc, but simply return true if a binding exists, and false if no bindings exist for the given key.
val mem_assq : 'a->map:('a * 'b) list-> bool
Same as mem_assoc, but uses physical equality instead of structural equality to compare keys.
val remove_assoc : 'a->('a * 'b) list->('a * 'b) list
remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not tail-recursive.
val remove_assq : 'a->('a * 'b) list->('a * 'b) list
Same as remove_assoc, but uses physical equality instead of structural equality to compare keys. Not tail-recursive.
Lists of pairs
val split : ('a * 'b) list->'a list * 'b list
Transform a list of pairs into a pair of lists: split [(a1,b1); ...; (an,bn)] is ([a1; ...; an], [b1; ...; bn]). Not tail-recursive.
val combine : 'a list->'b list->('a * 'b) list
Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is [(a1,b1); ...; (an,bn)].
if the two lists have different lengths. Not tail-recursive.
Sorting
val sort : cmp:('a->'a-> int)->'a list->'a list
Sort a list in increasing order according to a comparison function. The comparison function must return 0 if its arguments compare as equal, a positive integer if the first is greater, and a negative integer if the first is smaller (see Array.sort for a complete specification). For example, Stdlib.compare is a suitable comparison function. The resulting list is sorted in increasing order. sort is guaranteed to run in constant heap space (in addition to the size of the result list) and logarithmic stack space.
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.
val stable_sort : cmp:('a->'a-> int)->'a list->'a list
Same as sort, but the sorting algorithm is guaranteed to be stable (i.e. elements that compare equal are kept in their original order).
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.
val fast_sort : cmp:('a->'a-> int)->'a list->'a list
Same as sort or stable_sort, whichever is faster on typical input.
val sort_uniq : cmp:('a->'a-> int)->'a list->'a list
val merge : cmp:('a->'a-> int)->'a list->'a list->'a list
Merge two lists: Assuming that l1 and l2 are sorted according to the comparison function cmp, merge ~cmp l1 l2 will return a sorted list containing all the elements of l1 and l2. If several elements compare equal, the elements of l1 will be before the elements of l2. Not tail-recursive (sum of the lengths of the arguments).