[eeps] New BIFs for tuple arity and binary size

[Bjorn Gustavsson]

EEP: XXX
Title: New BIFs for tuple arity and binary size
Version: $Revision$
Last-Modified: $Date$
Author: Bjorn Gustavsson <bjorn@REDACTED>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 10-Aug-2007
Erlang-Version: R12B-0
Post-History: 

Abstract
========

This EEP describes the two new guards BIFs ``arity/1'' and ``bytesize/1''
as a prefered alternative to the ``size/1'' BIF.

Specifications
==============

``bytesize/1::binary() -> integer()``

Returns the size of a binary in bytes. This BIF will fail if
passed a *bitstr* (see http://www.erlang.org/eeps/eep-0004.html)
or any other type of term. This BIF is allowed in guards.

``arity/1::tuple() -> integer()``

Returns the arity for a tuple. This BIF will fail if passed anything
that is not a tuple. This BIF is allowed in guards.

Rationale
=========

The ``size/1'' BIF accepts either a binary or a tuple, and returns
either the size of binary in bytes or the arity of the tuple.

Because ``size/1'' accepts two different types, it is difficult to
optimize uses of it, both in the compiler and in the run-time system.
Adding the two new BIF will faciliate optimization, and will also
help Dialyzer.

Implementation
==============

The implementation is trivial.

Backwards Compatibility
=======================

Code containing local functions named ``arity/1'' or ``bytesize/1'' need
to be changed.

-- 
Björn Gustavsson, Erlang/OTP, Ericsson AB