# Module List

`module List = `struct ... end ``
 Functions

 ``` length``` : `'a list -> int`
Return the length (number of elements) of the given list.

 ``` hd``` : `'b list -> 'b`
Return the first element of the given list. Raise `Failure "hd"` if the list is empty.

 ``` tl``` : `'c list -> 'c list`
Return the given list without its first element. Raise `Failure "tl"` if the list is empty.

 ``` nth``` : `'d list -> int -> 'd`
Return the n-th element of the given list. The first element (head of the list) is at position 0. Raise `Failure "nth"` if the list is too short.

 ``` rev``` : `'e list -> 'e list`
List reversal.

 ``` append``` : `'f list -> 'f list -> 'f list`
Catenate two lists. Same function as the infix operator `@`. Not tail-recursive (length of the first argument). The `@` operator is not tail-recursive either.

 ``` rev_append``` : `'g list -> 'g list -> 'g list`
`List.rev_append l1 l2` reverses `l1` and catenates it to `l2`. This is equivalent to `List.rev l1 @ l2`, but `rev_append` is tail-recursive and more efficient.

 ``` concat``` : `'h list list -> 'h list`

 ``` flatten``` : `'i list list -> 'i list`
Catenate (flatten) a list of lists. Not tail-recursive (length of the argument + length of the longest sub-list).

 ``` iter``` : `f:('j -> unit) -> 'j list -> unit`
`List.iter f [a1; ...; an]` applies function `f` in turn to `a1; ...; an`. It is equivalent to `begin f a1; f a2; ...; f an; () end`.

 ``` map``` : `f:('k -> 'l) -> 'k list -> 'l list`
`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`. Not tail-recursive.

 ``` rev_map``` : `f:('m -> 'n) -> 'm list -> 'n list`
`List.rev_map f l` gives the same result as `List.rev (List.map f l)`, but is tail-recursive and more efficient.

 ``` fold_left``` : `f:('o -> 'p -> 'o) -> init:'o -> 'p list -> 'o`
`List.fold_left f a [b1; ...; bn]` is `f (... (f (f a b1) b2) ...) bn`.

 ``` fold_right``` : `f:('q -> 'r -> 'r) -> 'q list -> init:'r -> 'r`
`List.fold_right f [a1; ...; an] b` is `f a1 (f a2 (... (f an b) ...))`. Not tail-recursive.

 ``` iter2``` : `f:('s -> 't -> unit) -> 's list -> 't list -> unit`
`List.iter2 f [a1; ...; an] [b1; ...; bn]` calls in turn `f a1 b1; ...; f an bn`. Raise `Invalid_argument` if the two lists have different lengths.

 ``` map2``` : `f:('u -> 'v -> 'w) -> 'u list -> 'v list -> 'w list`
`List.map2 f [a1; ...; an] [b1; ...; bn]` is `[f a1 b1; ...; f an bn]`. Raise `Invalid_argument` if the two lists have different lengths. Not tail-recursive.

 ``` rev_map2``` : `f:('x -> 'y -> 'z) -> 'x list -> 'y list -> 'z list`
`List.rev_map2 f l` gives the same result as `List.rev (List.map2 f l)`, but is tail-recursive and more efficient.

 ``` fold_left2``` : `f:('a1 -> 'b1 -> 'c1 -> 'a1) -> init:'a1 -> 'b1 list -> 'c1 list -> 'a1`
`List.fold_left2 f a [b1; ...; bn] [c1; ...; cn]` is `f (... (f (f a b1 c1) b2 c2) ...) bn cn`. Raise `Invalid_argument` if the two lists have different lengths.

 ``` fold_right2``` : `f:('d1 -> 'e1 -> 'f1 -> 'f1) -> 'd1 list -> 'e1 list -> init:'f1 -> 'f1`
`List.fold_right2 f [a1; ...; an] [b1; ...; bn] c` is `f a1 b1 (f a2 b2 (... (f an bn c) ...))`. Raise `Invalid_argument` if the two lists have different lengths. Not tail-recursive.

 ``` for_all``` : `f:('g1 -> bool) -> 'g1 list -> bool`
`for_all p [a1; ...; an]` checks if all elements of the list satisfy the predicate `p`. That is, it returns `(p a1) && (p a2) && ... && (p an)`.

 ``` exists``` : `f:('h1 -> bool) -> 'h1 list -> bool`
`exists p [a1; ...; an]` checks if at least one element of the list satisfies the predicate `p`. That is, it returns `(p a1) || (p a2) || ... || (p an)`.

 ``` for_all2``` : `f:('i1 -> 'j1 -> bool) -> 'i1 list -> 'j1 list -> bool`

 ``` exists2``` : `f:('k1 -> 'l1 -> bool) -> 'k1 list -> 'l1 list -> bool`
Same as `for_all` and `exists`, but for a two-argument predicate. Raise `Invalid_argument` if the two lists have different lengths.

 ``` mem``` : `'m1 -> 'm1 list -> bool`
`mem a l` is true if and only if `a` is equal to an element of `l`.

 ``` memq``` : `'n1 -> 'n1 list -> bool`
Same as `mem`, but uses physical equality instead of structural equality to compare list elements.

 ``` find``` : `f:('o1 -> bool) -> 'o1 list -> 'o1`
`find p l` returns the first element of the list `l` that satisfies the predicate `p`. Raise `Not_found` if there is no value that satisfies `p` in the list `l`.

 ``` filter``` : `f:('p1 -> bool) -> 'p1 list -> 'p1 list`

 ``` find_all``` : `f:('q1 -> bool) -> 'q1 list -> 'q1 list`
`filter p l` returns all the elements of the list `l` that satisfy the predicate `p`. The order of the elements in the input list is preserved. `find_all` is another name for `filter`.

 ``` partition``` : `f:('r1 -> bool) -> 'r1 list -> 'r1 list * 'r1 list`
`partition p l` returns a pair of lists `(l1, l2)`, where `l1` is the list of all the elements of `l` that satisfy the predicate `p`, and `l2` is the list of all the elements of `l` that do not satisfy `p`. The order of the elements in the input list is preserved.

 ``` assoc``` : `'s1 -> ('s1 * 't1) list -> 't1`
`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`. Raise `Not_found` if there is no value associated with `a` in the list `l`.

 ``` assq``` : `'u1 -> ('u1 * 'v1) list -> 'v1`
Same as `assoc`, but uses physical equality instead of structural equality to compare keys.

 ``` mem_assoc``` : `'w1 -> ('w1 * 'x1) list -> bool`
Same as `assoc`, but simply return true if a binding exists, and false if no bindings exist for the given key.

 ``` mem_assq``` : `'y1 -> ('y1 * 'z1) list -> bool`
Same as `mem_assoc`, but uses physical equality instead of structural equality to compare keys.

 ``` remove_assoc``` : `'a2 -> ('a2 * 'b2) list -> ('a2 * 'b2) list`
`remove_assoc a l` returns the list of pairs `l` without the first pair with key `a`, if any. Not tail-recursive.

 ``` remove_assq``` : `'c2 -> ('c2 * 'd2) list -> ('c2 * 'd2) list`
Same as `remove_assq`, but uses physical equality instead of structural equality to compare keys. Not tail-recursive.

 ``` split``` : `('e2 * 'f2) list -> 'e2 list * 'f2 list`
Transform a list of pairs into a pair of lists: `split [(a1,b1); ...; (an,bn)]` is `([a1; ...; an], [b1; ...; bn])`. Not tail-recursive.

 ``` combine``` : `'g2 list -> 'h2 list -> ('g2 * 'h2) list`
Transform a pair of lists into a list of pairs: `combine ([a1; ...; an], [b1; ...; bn])` is `[(a1,b1); ...; (an,bn)]`. Raise `Invalid_argument` if the two lists have different lengths. Not tail-recursive.

 ``` sort``` : `cmp:('i2 -> 'i2 -> int) -> 'i2 list -> 'i2 list`
Sort a list in increasing order according to a comparison function. The comparison function must return 0 if it arguments compare as equal, a positive integer if the first is greater, and a negative integer if the first is smaller. For example, the `compare` function is a suitable comparison function. The resulting list is sorted in increasing order. `List.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 and is the same as `List.stable_sort`.

 ``` stable_sort``` : `cmp:('j2 -> 'j2 -> int) -> 'j2 list -> 'j2 list`
Same as `List.sort`, but the sorting algorithm is stable.
The current implementation is Merge Sort. It runs in constant heap space and logarithmic stack space.