Author: James Hague <james(dot)hague(at)gmail(dot)com>
    Status: Draft
    Type: Standards Track
    Created: 18-Feb-2009
    Post-History:
****
EEP 27: Multi-Parameter Typechecking BIFs
----

Abstract
========

Typechecking guards (e.g., `is_float/1`) are useful for a number of
reasons, but they're verbose.  I propose allowing multiple
parameters to the `is_` famility of functions, which
significantly reduces source code bulk in common cases.

Specification
=============

Where `is_type` represents any of the `is_` family of functions,
such as `is_float`:

`is_type(A, B, C, ...)` is equivalent to `(is_type(A) andalso
is_type(B) andalso is_type(C)...)`.

The is_type functions can now take from 1 to N parameters, where
N is the implementation defined limit on function arity.

The old-style guards (e.g., `float/1`) would not change, as some of
those serve double duty as typecasts.

Direct references to these functions in the erlang module are for
the single parameter versions only (such as `fun
erlang:is_float/1`).

Motivation
==========

I find myself adding typechecking guards not only for safety, but
to improve code generation quality, especially when using floats.
Writing three or four element vector math functions in Erlang,
with `is_float` guards, is verbose.  The `is_float` checks dwarf what
would otherwise be a single-line function by adding multiple lines
of guards.

Rationale
=========

Here's an example from the Wings3D project:

    cross({V10,V11,V12}, {V20,V21,V22})
      when is_float(V10), is_float(V11), is_float(V12),
           is_float(V20), is_float(V21), is_float(V22) ->
        {V11*V22-V12*V21,V12*V20-V10*V22,V10*V21-V11*V20}.

The `is_float` checks significantly improve the quality of the
generated code, allowing floats to be kept in virtual machine
registers instead of allocated on the heap.  If multiple
parameters to `is_float` were allowed, this code could be
rewritten as:

    cross({V10,V11,V12}, {V20,V21,V22})
      when is_float(V10,V11,V12,V20,V21,V22) ->
        {V11*V22-V12*V21,V12*V20-V10*V22,V10*V21-V11*V20}.

In the second version, the intent is clearer at a glance, and
the source-level weight of adding typechecking doesn't overwhelm
the function.

Over the years the the Erlang system has become more reliant on
typechecking.  There are the dialyzer and typer tools.  The
compiler can statically infer types and generate better code as
a result. Making typechecking guards be lighter-weight at the
source code level encourages their use and is more in-line with
the overall syntactic density of the language.

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

All uses of the `is_type/1` functions will still work if this
proposal were implemented.  Direct references to
`erlang:is_float`, `erlang:is_atom`, etc., as funs will still work
as originally intended.

Reference Implementation
========================

None.

Copyright
=========

This document has been placed in the public domain.

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