The Standard ML Basis Library

The `BIT_FLAGS` signature

Synopsis

signature BIT_FLAGS (* OPTIONAL *)

The BIT_FLAGS signature defines a generic set of operations on an abstract representation of system flags. It is typically included as part of the interface of substructures which provide a set of options.

Interface

eqtype flags val toWord : flags -> SysWord.word val fromWord : SysWord.word -> flags val all : flags val flags : flags list -> flags val intersect : flags list -> flags val clear : flags * flags -> flags val allSet : flags * flags -> bool val anySet : flags * flags -> bool

Description

eqtype flags

This type is the abstract representation of a set of system flags.

val toWord : flags -> SysWord.word val fromWord : SysWord.word -> flags

These functions convert between the abstract flags type and a bit-vector that is represented as a system word. The interpretation of the bits is system-dependent, but follows the C language binding for the host operating system. Note that there is no error checking on the fromWord function's argument.

all

represents the union of all flags. Note that this may well be a superset of the flags value defined in a matching structure. For example, BIT_FLAGS is used to define the flags specified by the POSIX standard; a POSIX-conforming operating system may provide additional flags that will not be defined in the Posix structure but could be set in the all value.

flags l

returns a value that represents the union of the flags in the list l. The expression flags [] denotes the empty set.

intersect l

returns a value that represents the intersection of the sets of flags in the list l. The expression intersect [] denotes all.

clear (fl1, fl2)

returns the set of those flags in fl2 that are not set in fl1, i.e., the set difference fl2 \ fl1. It is equivalent to:

fromWord(SysWord.andb(SysWord.notb (toWord fl1), toWord fl2))

allSet (fl1, fl2)

returns true if all of the flags in fl1 are also in fl2 (i.e., this tests for inclusion of fl1 in fl2).

anySet (fl1, fl2)

returns true if any of the flags in fl1 is also in fl2 (i.e., this tests for non-empty intersection).

Discussion

The number of distinct flags in an implementation of the BIT_FLAGS interface must be less than or equal to the number of bits in the SysWord.word type. In addition, fromWord o toWord must be the identity function, and toWord o fromWord must be equivalent to

fn w => SysWord.andb(w, toWord all)

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Generated April 12, 2004
Last Modified July 1, 2002
Comments to John Reppy.

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