Lambda vs. function

Dave Herman dherman at ccs.neu.edu
Tue Oct 28 11:55:07 PDT 2008


>> Specifically, in the tail-calling cases:
>>
>>    g(); break;
>>
>> has the unintuitive behavior of *first* breaking out of the 
>> switch-block and *then* evaluating g().
> 
> Since the difference cannot be observed (other than through growth of 
> the control stack), I don't see that it really matters which is done 
> first. Otherwise, many common compiler optimizations (instruction 
> scheduling, common subexpression elimination, load/store hoisting...) 
> are uninituitive by the same standard.

That's a fair point. I guess I'll stand down on this.

I do want to stress that we're not talking about tail calls as an 
optimization, but as a requirement of the language. The programmer needs 
to be able to rely on them, and so it needs to be well-specified exactly 
where the tail positions are. If that specification is complicated, it 
could lead to refactoring hazards, and the resulting stack growth (or 
overflow) could be hard to debug.

I guess my feeling is that the notion of tail position is just cleaner 
when it doesn't involve control operators like `break' and `return', and 
that's one of the reasons I like the idea of a `lambda' form that 
doesn't use `return'.

But I won't argue against having more tail positions in forms like 
`switch'. So much the better, even if it's messy.

Dave



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