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Source for wiki WG1Ballot8Medernach version 1

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medernac

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Backup of 8th Ballot

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88.177.167.25

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WG1Ballot8Medernach

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== WG1 - Core ==

=== #121 The semantics of expt for zero bases has been refined ===

The R5RS definition of expt is:

{{{
 -- procedure: expt z1 z2
     Returns Z1 raised to the power Z2.  For z_1 ~= 0

                          z_1^z_2 = e^z_2 log z_1

     0^z is 1 if z = 0 and 0 otherwise.
}}}

however exponents with negative real parts are undefined.
R6RS attempted to clarify this with:

{{{
     0.0^z is 1.0 if z = 0.0, and 0.0 if (real-part z) is positive.
     For other cases in which the first argument is zero, either
     an error is signalled or an unspecified number is returned.
}}}

(Ignore the change in exactness, which was strictly editorial
and the examples clarify that the rules ignore exactness.)

This is unique in all the reports of a result either
signalling an error or returning a value.  The motivation
for this was because R6RS consistently removed uses of the
"is an error" terminology which would more naturally fit
this situation.

An alternative, `r5rs-error`, is to restore the "is an error"
text since we are not avoiding this in R7RS:

{{{
     The value of 0^z is 1 if (zero? z), 0 if (real-part z)
     is positive, and an error otherwise.  Similarly for 0.0^z,
     with inexact results.
}}}

The `/real` variant restricts the domain for the zero
base exception to the real numbers.  This is because
0^z^ is mathematically undefined for non-real z, and
implementations do not agree on the result.

  * '''Options:''' r5rs, r5rs-error, r5rs-error/real, r6rs, r6rs/real, undecided
  * '''Default:''' r6rs
  * '''Preferences:''' r5rs-error, r5rs, undecided, r5rs-error/real, r6rs, r6rs/real

As I understand the above text is just false: 0^0^ and 0.0^0.0^ are mathematicaly undefined, this is because it is not continuous there. Just take x^(-1/log(x))^, when x -> 0 it is equal (and therefore converges) to 1/e instead of 1 ! Provided this is changed I prefer the openness of r5rs-error.
Bradley's argument convinces me to retain 0^0^=1 (i.e. only if we have an exact 0 as exponent) as a practical convention.


=== #472 clarify semantics of non-library library declarations ===

In items #91, #148 and #150 we voted to allow the
use of `include`, `include-ci` and `cond-expand`
at the "top-level" respectively, but there remains
some confusion as to their semantics.

Here "top-level" refers to repl and program body
top-levels, but not library bodies.

One interpretation is that these behave like library
declarations, and can expand into `import` forms.
In this case, for a purely static implementation of
R7RS libraries, they must first be statically scanned
from all top-level forms.  They cannot be used
outside the top-level, and are not even available
as bindings otherwise.  This is the `declaration`
proposal.

Another interpretation is that they are just normal
macros with the obvious definitions (cond-expand
in terms of the output of the `features` macro),
are available in the `(scheme base)` library, and
consequently can't be used to expand into `import`
since imports have already been resolved.  This is
the `syntax` proposal.

Alternately, we could provide `both`.  If you think
this is all too confusing you could also vote `remove`,
to drop these extensions.

  * '''Options:''' declaration, syntax, both, remove
  * '''Default:''' 
  * '''Preferences:''' syntax, remove, declaration, both

=== #473 library declaration locations in top-level ===

R6RS allows only a single library declaration, `import`,
at the beginning of a program body, and this must
contain all imported libraries.

Pending the result of ticket #472 we may also allow
`include(-ci)` and `cond-expand` to expand into
imports, and so the single form restriction would not
make sense.  However, it would be reasonable to
restrict all library declarations to the beginning of
a program - the first non-declaration would begin
the real body.  This is the `beginning-only` option.

The advantage of the `r6rs` proposal is that it would
not require any changes in existing R6RS program
loading implementations to support.  If the result of
ticket #472 indicates multiple declaration types are
available this option would automatically become
invalid, so you don't need to vote against it on those
grounds.

The advantage of the `beginning-only` option is
that it becomes possible to statically determine
all program imports without expansion, which was
the primary motivation of a static library system.

The final alternative is `any-top-level`, which
allows these forms anywhere at the top-level,
possibly interspersed with definitions.  The advantage
of this is that you can cut&paste repl sessions
(for which interspersed imports are always allowed)
as a program.  The disadvantage is that programs
can no longer be resolved separately from expansion.

  * '''Options:''' r6rs, beginning-only, any-top-level
  * '''Default:''' 
  * '''Preferences:'''  beginning-only, r6rs, any-top-level

=== #405 Retract language requiring force to accept non-promises ===

#405 lumped together several issues, one of which was a requirement
(as opposed to an option) to make `force` applied to a non-promise
return its argument, as opposed to it being an error.  Thus, it would
require `(force 2) => 2`.  However, R6RS
requires `(force 2)` to signal an error, and many non-R6RS Schemes also
signal an error (see ForceNonPromise for details).  These facts were not
considered at the time.

Vote `retain` to retain this requirement, or `retract` to retract it
and leave the result of `(force 2)` implementation-dependent.

  * '''Options:''' retain, retract
  * '''Default:''' retain
  * '''Preferences:''' retract

time

2012-12-02 23:43:24

version

1