# Structure ListPair

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```(* ListPair -- SML Basis Library *)

val zip    : 'a list * 'b list -> ('a * 'b) list
val unzip  : ('a * 'b) list -> 'a list * 'b list
val map    : ('a * 'b -> 'c)   -> 'a list * 'b list -> 'c list
val app    : ('a * 'b -> unit) -> 'a list * 'b list -> unit
val all    : ('a * 'b -> bool) -> 'a list * 'b list -> bool
val exists : ('a * 'b -> bool) -> 'a list * 'b list -> bool
val foldr  : ('a * 'b * 'c -> 'c) -> 'c -> 'a list * 'b list -> 'c
val foldl  : ('a * 'b * 'c -> 'c) -> 'c -> 'a list * 'b list -> 'c

(*
These functions process pairs of lists.  No exception is raised
when the lists are found to be of unequal length.  Instead the
excess elements from the longer list are disregarded.

[zip (xs, ys)] returns the list of pairs of corresponding elements
from xs and ys.

[unzip xys] returns a pair (xs, ys), where xs is the list of first
components of xys, and ys is the list of second components from
xys.  Hence zip (unzip xys) has the same result and effect as xys.

[map f (xs, ys)] applies function f to the pairs of corresponding
elements of xs and ys and returns the list of results.  Hence
map f (xs, ys) has the same result and effect as List.map f (zip (xs, ys)).

[app f (xs, ys)] applies function f to the pairs of corresponding
elements of xs and ys and returns ().  Hence app f (xs, ys) has the
same result and effect as List.app f (zip (xs, ys)).

[all p (xs, ys)] applies predicate p to the pairs of corresponding
elements of xs and ys until p evaluates to false or one or both
lists is exhausted; returns true if p is true of all such pairs;
otherwise false.  Hence all p (xs, ys) has the same result and
effect as List.all p (zip (xs, ys)).

[exists p (xs, ys)] applies predicate p to the pairs of corresponding
elements of xs and ys until p evaluates to true or one or both
lists is exhausted; returns true if p is true of any such pair;
otherwise false.  Hence exists p (xs, ys) has the same result and
effect as List.exists p (zip (xs, ys)).

[foldr f e (xs, ys)] evaluates f(x1, y1, f(x2, y2, f(..., f(xn, yn, e))))
where xs = [x1, x2, ..., x(n-1), xn, ...],
ys = [y1, y2, ..., y(n-1), yn, ...],
and    n = min(length xs, length ys).
Equivalent to List.foldr (fn ((x, y), r) => f(x, y, r)) e (zip(xs, ys)).

[foldl f e (xs, ys)] evaluates f(xn, yn, f( ..., f(x2, y2, f(x1, y1, e))))
where xs = [x1, x2, ..., x(n-1), xn, ...],
ys = [y1, y2, ..., y(n-1), yn, ...],
and    n = min(length xs, length ys).
Equivalent to List.foldl (fn ((x, y), r) => f(x, y, r)) e (zip(xs, ys)).
*)

```

Identifier index Structure index

Moscow ML 2.00