Explain more on benefits/use-case of using is pure trait
#1231
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Can somebody review what I just did? Does it resolve the issue? |
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Yep, makes it very clear. |
moritz
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Sep 3, 2017
explain why the compiler cannot reliably infer purity on its own. For issue #1231.
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I don't pretend to reopen this issue, and what I'm about to say is not
specific to `is pure`.
Wouldn't it be better to look for actual use cases and insert them as
examples? While multiplying input by 2 is syntactically correct, it's
relatively unlikely to meet it in a real life situation; you will simply
multiply by 2 without needing to create a function. Not that examples
elsewhere are better:
https://medium.com/@jamesjefferyuk/javascript-what-are-pure-functions-4d4d5392d49c
but this
http://www.deadcoderising.com/2017-06-13-why-pure-functions-4-benefits-to-embrace-2/
for instance mentions composition, laziness, both of them having support in
perl 6, and maybe they could be mentioned here in this example.
Since I do have write permission here, would you like me to try something
here along these lines? Just reopen the issue and assign it to me if you
want.
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@JJ Perl 6 can have composition, laziness and concurrency on normal functions just fine. The trait is used to change their performance characteristics, moving a runtime calculation (which could be cached at runtime too: https://docs.perl6.org/routine/is%20cached) into compilation time. |
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@JJ feel free to provide a better example |
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2017-09-03 10:24 GMT+02:00 rafaelschipiura <notifications@github.com>:
@JJ <https://github.com/jj> Perl 6 can have composition, laziness and
concurrency on normal functions just fine. The trait is used to change
their performance characteristics, moving a runtime calculation (which
could be cached at runtime too: https://docs.perl6.org/routine/is%20cached)
into compilation time.
I didn't mean those were *exclusive* traits of pure functions. My point was
that it would be interesting to provide meaningful use cases by providing
examples that fall in them.
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@JJ I agree your idea is good, I was just trying to help find a good example. |
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2017-09-03 10:28 GMT+02:00 rafaelschipiura <notifications@github.com>:
@JJ <https://github.com/jj> I agree your idea is good, I was just trying
to help find a good example.
Thanks. In fact, caching/memoization is another use case of pure routines.
I'll try to find examples for all of them, maybe run some measurements too.
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@JJ I just want you to note that the very example provided in the documentation has side effects and Perl6 has no problem with it, OK? |
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I've come with a nice use case that includes pre-computing a set of things; however it involves using experimental macros. Would this be suitable? Or is it better if I think about a different one? |
No. |
👎 even if they weren't experimental. There's no need to daunt the user with a whole 'nother large language feature they'd have to be familiar with just to explain
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Correct me if I'm wrong, but only if you call them with constants they will
be evaluated in compile time. You can obviously call them with a variable,
but their value will not be available in compile time, so they will act as
"normal" variables.
That's what I mean when I say a "real" use case. It could be a difficult
computation performed in compile time, for instance, or pre-computing some
calls. That is also why I say we would need a macro to have an use case as
real as possible.
I was thinking, for instance, about a problem we often find in evolutionary
algorithms, royal road. It's computed with 4 bits, you can pretty much put
it in a hash or else write a macro that generates its 16 cases, which then
use the "is pure". Programatically it will not look like a sequence of 16
function calls, but a loop, and the 16 cases will be actually computed in
compile time.
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For instance, this thing: four-bits() is called at compile time. However, royal-road is only called at compile time for the "1010" call. A macro could generate the code instead of the map. |
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https://docs.perl6.org/routine/is%20pure
Even after reading about what constant-folding is, it's not clear why someone would need to explicitly mark this when the compiler can probably infer it on its own.
Is there a difference between using
is pureon a sub that has a return value vs on a sub that doesn't have a return value?The text was updated successfully, but these errors were encountered: