> On Feb 16, 2017, at 3:13 PM, David Hart via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Now that I've thought more about it, I have a question. Escaping/unescaping
> is an important concept to have in the language: if the API provider makes
> the promise that a closure is non-escaping, the API client doesn't have to
> worry about the closure capturing variables and creating strong references to
> them.
>
> But in the case of pure functions, I fail to get the benefit from the
> examples. Could someone explain to me the advantages with a more in-depth
> example?
AFAIK, the two most direct benefits are memoization (without an “r”) and
concurrency. Because the everything the function does is contained within the
function itself and it can’t change or be affected by any global state, the
compiler can prove it’s ok to, for instance, parallelize a loop for you if
everything inside the loop is “pure". Additionally, because a pure function
will always give you the same output for the same input, you (or the compiler,
if it supports that level of optimization) can safely cache the results of a
function call in a Dictionary or something, and next time the function is
called do a quick lookup to see if you already know the answer before passing
the inputs along to the “real" function. The benefits of this obviously depend
on what the function is doing… it wouldn’t make sense to cache simple integer
addition, but if your function has, IDK, loops nested 23 levels deep and they
all involve floating point division and matrix multiplication or something,
caching the answer will likely pay off.
Here’s a article/site/blog/etc that talks about the more “quality of life” kind
of benefits: http://scalafp.com/book/benefits-of-pure-functions.html (note that
some people will vigorously contest the “easier to reason about” claim)
- Dave Sweeris
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