Formal comment #188 (enhancement)

Add symbol=? and boolean=?
Reported by: 	Lauri Alanko
Version:	5.92

Email: la at iki.fi
Summary

Dedicated equivalence predicates for symbols and booleans.

Description

I propose adding to the base library two new equivalence predicates
whose arguments are restricted to symbols and booleans.

One reason for this is simply consistency. It is customary for all
(non-singleton) types to have their own dedicated equivalence
predicates. Symbols and booleans seem to be the only exceptions.

The benefits of dedicated predicates would be the same as usual: in
contexts where the arguments are expected to be of a single fixed
type, they provide more information to the programmer and the
compiler, and better error checking. Especially symbol-only
comparisons are very common.

Currently, symbol comparisons are often done with eq?, which seems
like bad style, since eq? is a low-level optimization primitive with
implementation-dependent semantics. Providing an alternative would
hopefully encourage writing better-defined code.

I get the feeling that the reason for the lack of the predicates is
that symbols and booleans are already guaranteed to be comparable with
eq?, so the implementations cannot be anything but trivial. However, I
think the guarantees made for an optimization loophole shouldn't
affect the design of the rest of the language.

PLT Scheme's MzLib? library currently provides these predicates, and
they seem to enjoy some minor usage.

Proposal

Add to Section 9.10, page 46, the following:

(boolean=? boolean1 boolean2)                      procedure

    Returns #t if boolean1 and boolean2 are both #t or both #f,
    otherwise returns #f.

Add to Section 9.12, page 49, the following:

(symbol=? symbol1 symbol2)                         procedure

    Returns #t if symbol1 and symbol2 have the same name, otherwise
    returns #f.

In Section 9.6, page 37, change the definition of eqv? for symbols and
booleans to refer to the above procedures.


RESPONSE:

The suggestion will be adopted in the next draft of the report.