[erlang-questions] per function/process locals
Wed May 2 03:36:44 CEST 2007
On 2 May 2007, at 8:44 am, James Hague wrote,
about tail recursive loops with many arguments simulating
simple imperative variables:
> I actually do this quite a bit.
Why? Is there an example you can provide?
> Even just having 5 values
> getting passed around and "updated" in a loop bugs me.
Updating 5 variables in a C loop would bug me, most of the time.
> I think the Erlangyest solution is to implement Joe's structs proposed
> a few years ago, then have the compiler recognize situations where
> structs are created and modified in a loop.
If the compiler recognised and special-cased similar code involving
tuples, then the existing records would do the job just as well.
There are reasons to expect this optimisation to be easier than the
>> I'd rather have the let-for-then construct.
> Could you give an example?
A quick search for 'let then for in fold' in the Erlang mailing list
archive found this:
... we had a thread about looping not so very long ago.
By the end of it, there was a concrete proposal for a construct looking
(let p1 = i1 then s1, ..., pk = ik then sk
[Added in May 2007: "p" suggests "pattern", "i" suggests "initial
"s" suggests "step", generators-and-filters is whatever can currently
follow "||" in a list comprehension, final-result is an expression.]
As a trivial example, let's compute the mean of the positive elements
of a list:
(let N = 0 then N+1, S = 0 then S+X
for X <- List, X > 0
It's pretty much what you get if you hybridise Scheme DO loops with
list comprehension, and it's as pure as anyone could wish for.
[Added in May 2007: best of all, it doesn't add a new kind of "variable"
to the language. Nor does it require heavy-duty optimisation to recover
the intended structure. The only tricky thing is the rebinding of the
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