On Tue, Jun 04, 2019 at 07:09:52AM -0600, Matthew Flatt wrote:
> Thanks for all the replies about single-flonum uses!
>
> I've pushed the change to try out disabling single-flonum literals as
> of v7.3.0.5.
>
> Note that this change doesn't remove the concept of single-flonum
> values from the language. It just removes single-flonum literals from
> `#lang racket` and other languages that use the S-expression reader.
>
>
> If you have a program with a single-flonum literal expression in it,
> such as
>
> 3.4f5
>
> then you can convert to a use of `real->signle-flonum`:
>
> (real->single-flonum 3.4e5)
>
> On a Racket variant that supports single-flonum values (like the
^^^^^^^^
> current version of Racket), the compiler will constant-fold that
> expression to a single-flonum value, so the compiled code is the same
> as writing a single-flonum literal. On a Racket variant that does
> support single-flonum values, however, that expression will raise an
^^^^^^^
> exception.
Should one of these "support"s be preceded by "not"?
>
> Here's an example in `degrees->radians` in `racket/math`:
>
> https://github.com/racket/racket/blob/master/racket/collects/racket/math.rkt#L97
>
> The call to `real->single-flonum` is in a `cond` clause that is guarded
> with a `single-flonum?` test. Obviously, that guard will succeed only
> in a Racket variant that supports single-flonum values, so we don't
> need to worry about the `real->single-flonum` operation failing.
>
> For cases where there's no natural `single-flonum?` guard, a new
> `single-flonum-available?` function reports whether single-flonum
> values are supported. It currently produces #t in the current version
> of Racket and #f in Racket CS.
>
> Finally, you can set the new `read-single-flonum` parameter to #t to
> restore single-flonum parsing at the level of `read`. You can set
> `read-single-flonum` to #t on Racket variant that does not support
> single-flonum values, but `read` will raise an exception if it
> encounters a single-flonum number.
>
> Although we could make a language or a language constructor that
> enables single-flonum literals by setting `real-single-flonum`, I think
> we should try discouraging that, for now. Admittedly, I used a little
> reader to do that in the test suite for the core single-flonum
> operations, but that feels like a special case.
>
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