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package travesty
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Generic_extensions refers to the container type as 'a t, and the element type as 'a elt; substitute t/elt (arity-0) or 'a t/'a (arity-1) accordingly below.

include Types_intf.Generic
type 'a t

Placeholder for the container type.

type 'a elt

Placeholder for the type of elements inside the container.

Testing for a specific number of elements

The following functions help in checking whether a container has a particular, commonly-required number of elements (zero or one, one, two, and so on).

val at_most_one : 'a t -> 'a elt Base.option Base.Or_error.t

at_most_one xs returns Ok None if xs is empty; Ok Some(x) if it contains only x; and an error otherwise.

Examples (using an extended version of List):

List.at_most_one []     (* ok None *)
     at_most_one [1]    (* ok (Some 1) *)
     at_most_one [1; 2] (* error -- too many *)
val one : 'a t -> 'a elt Base.Or_error.t

one xs returns Ok x if xs contains only x, and an error otherwise.

Examples (using an extended version of List):

List.one []     (* error -- not enough *)
     one [1]    (* ok 1 *)
     one [1; 2] (* error -- too many *)
val two : 'a t -> ('a elt * 'a elt) Base.Or_error.t

two xs returns Ok (x, y) if xs is a list containing only x and y in that order, and an error otherwise.

Examples (using an extended version of List):

List.two []        (* error -- not enough *)
     two [1]       (* error -- not enough *)
     two [1; 2]    (* ok (1, 2) *)
     two [1; 2; 3] (* error -- too many *)

Miscellaneous extensions

val max_measure : measure:('a elt -> Base.int) -> ?default:Base.int -> 'a t -> Base.int

max_measure ~measure ~default xs measures each item in xs according to measure, and returns the highest measure reported. If xs is empty, return default if given, and 0 otherwise.