# SML/NJ Library Manual

## The `ORD_KEY` signature

#### Synopsis

> `signature `ORD_KEY`
> `

The ORD_KEY signature provides an abstract description of ordered keys. It specifies a type plus a trivalent comparison function.

#### Interface

> `type ord_key
> val compare : (ord_key * ord_key) -> order
> `

#### Description

`type ord_key`
`compare (key, key')`
  returns LESS, EQUAL, or GREATER if and only if key is less than, equal to, or greater than key', respectively, in some defined linear ordering.

#### See Also

> ORD_MAP, ORD_SET, BinarySetFn, BinaryMapFn, SplaySetFn, SplayMapFn, ListSetFn, ListMapFn

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Last Modified June 5, 1998
Comments to John Reppy
Copyright © 1998 Bell Labs, Lucent Technologies