[r6rs-discuss] [Formal] preposterous mustard
sperber at informatik.uni-tuebingen.de
Mon Feb 5 07:18:57 EST 2007
William D Clinger <will at ccs.neu.edu> writes:
> Page 35, specification of letrec*:
> One restriction on letrec* is very important: it must be possible
> to evaluate each init without assigning or referring to the value
> the corresponding variable or the variable of any of the bindings
> that follow it in bindings.
> It is undecidable whether there exists an order of evaluation
> for the right hand sides of the bindings that can avoid such
> assignments or references. The R6RS should permit implementations
> to evaluate expressions in some particular order, without
> requiring implementations to perform an exhaustive search of
> the order-of-evaluation space at run time, complete with the
> overhead necessary to undo any side effects that are performed
> by evaluations that run afoul of the restriction.
I see your point for `letrec', which you describe separately.
However, for `letrec*' order of evaluation is specified. So the
comment about "exhaustive search of the order-of-evaluation space"
doesn't seem to apply. Am I missing something?
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla
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