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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