\DOC insert
\TYPE {insert ''a -> ''a list -> ''a list}
\SYNOPSIS
Add an element to a list if it is not already there.
\KEYWORDS
list, set, eqtype.
\DESCRIBE
If {x} is already in {list}, then {insert x list} equals {list}.
Otherwise, {x} becomes an element of {list}.
\FAILURE
Never fails.
\EXAMPLE
{
- insert 1 [3,2];
> val it = [1, 3, 2] : int list
- insert 1 it;
> val it = [1, 3, 2] : int list
}
\COMMENTS
In some programming situations, it is convenient to implement sets
by lists, in which case {insert} may be helpful. However, such
an implementation is only suitable for small sets. Serious
implementations of sets may be found in the Standard ML Basis Library.
ML equality types are used in the implementation of {insert} and its kin.
This limits its applicability to types that allow equality. For other
types, typically abstract ones, use the `op_' variants.
One should not write code that depends on where the `list-as-set'
algorithms place elements in the list which is being considered as a
set.
\SEEALSO
Lib.op_insert, Lib.mem, Lib.mk_set, Lib.union, Lib.U, Lib.set_diff,
Lib.subtract, Lib.intersect, Lib.null_intersection, Lib.set_eq.
\ENDDOC