Re: [Scheme-reports] multiple values, and, or, when, unless

[Alex Shinn <alexshinn@x>]

Please forget I ever mentioned dropping the TCO requirement.

On Sun, Jan 6, 2013 at 10:11 PM, Jussi Piitulainen <jpiitula@x> wrote:

> The tail call is an orthogonal issue, let's please forget it.

Surely it's not an orthogonal issue. The values of an _expression_ in a
tail position are passed to the continuation. But as I said, I'm fine
as long as I can have the bounded-space behaviour.

You're still confusing the implementation with the specification.  

All that TCO guarantees is that you won't run out of space.

There are many ways to guarantee this.

Consider an implementation which translates when to

  (if test (begin expr ... (if #f #f)))

when the implementation uses Cheney on the MTA plus

an optimization to compact chains of continuations passing

constant unspecified values to just pass the final value.

This will never run out of space.

Moreover, there's a difference between something being

always implied in practice and being required by the

specification.  If the standard leaves the result of when

unspecified, compilers are free to optimize:

  (when test

    (foo)

    (expensive-referentially-transparent-computation))

to just

  (when test

    (foo))

but are not free to do so if we specify the result.

-- 

Alex

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