[eeps] Multi-Parameter Typechecking BIFs

[James Hague]

See attachment.
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EEP: XXX
Title: Multi-Parameter Typechecking BIFs
Version: $Revision: 37 $
Last-Modified: $Date: 2008-07-11 15:47:13 +0200 (Fri, 11 Jul 2008) $
Author: James Hague [james.hague@REDACTED]
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 18-Feb-2009
Post-History:


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.

References
    
    None.

Copyright

    This document has been placed in the public domain.



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