'Use-Once' Variables and Linear Objects
nagle at animats.com
Sat Aug 13 06:30:28 CEST 2011
On 8/2/2011 7:19 AM, Neal Becker wrote:
> I thought this was an interesting article
Single-use was something of a dead end in programming.
Single assignment, where you can only set a variable when you create
it, is more useful. Single assignment is comparable to functional
programming, but without the deeply nested syntax.
Functional programs are trees, while single-assignment programs are
directed acyclic graphs. The difference is that you can fan-out
results, while in a a functional language, you can only fan in.
This fits well with Python, where you can write things like
def fn(x) :
(a, b, c) = fn1()
return(fn2(a) + fn3(b)*c)
"const" is often used in C and C++ to indicate single-assignment usage.
But C/C++ doesn't have multiple return values, so the concept isn't as
useful as it is in Python.
Optimizing compilers usually recognize variable lifetimes, and so they
create single-assignment variables internally when possible. This
is a win for register and stack allocation, and for fine-grain
parallelism on machines which support it.
Since Python isn't very optimizable, this is mostly a curiosity.
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