[ruby-core:101162] [Ruby master Feature#17316] On memoization

Issue #17316 has been updated by Dan0042 (Daniel DeLorme).


marcandre (Marc-Andre Lafortune) wrote in #note-3:
> Gem is here: https://github.com/marcandre/ractor-cache
> Comments welcome :-)

Since you say so... :-)
An additional strategy might to wrap the @cache in a Ractor::LVar (if/once available). I tend to use memoization to cache DB access rather than long calculations, and for a given class I would probably not use all (or even a majority) of memoized methods at once. So pre-computing values before deep-freezing is not a good option for me.

But I find it interesting that this memoization stuff keeps getting reimplemented.
https://rubygems.org/search?utf8=%E2%9C%93&query=memoization
Not to mention all the people (including me) who have implemented this in their private code.
And everyone tends to have a slightly different implementation based on the features they need.

For example my own implementation is compatible with shallow-freezing and falsy values, but not with methods that take arguments; instead I wanted cache-busting based on dependent values. And multiple-assignment aliases.

```ruby
memo ->{id}, #memo-busting lambda
:foo, :bar,  #aliases for foobar[0] and foobar[1]
def foobar
  obj = get_foobar_from_db(id)
  [obj.foo, obj.bar]
end
```

All this to say that since the specifics can vary, it's probably better to leave that level of memoization to gems and individual developers. I can somewhat agree with something simple like `instance_variable_get(:@v){ @v = calc() }` ... but then again we can already do this just as easily now with `return @v if defined? @v; @v = calc()`


----------------------------------------
Feature #17316: On memoization
https://bugs.ruby-lang.org/issues/17316#change-88848

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
I have seen so many attempts to memoize a value in the form:

```ruby
@foo ||= some_heavy_calculation(...)
```

improperly, i.e., even when the value can potentially be falsy. This practice is wide spread, and since in most cases memoization is about efficiency and it would not be critical if it does not work correctly, people do not seem to care so much about correcting the wrong usage.

In such case, the correct form would be:

```ruby
unless instance_variable_defined?(:@foo)
  @foo = some_heavy_calculation(...)
end
```

but this looks too long, and perhaps that is keeping people away from using it.

What about allowing `Kernel#instance_variable_set` to take a block instead of the second argument, in which case the assignment should be done only when the instance variable is not defined?

```ruby
instance_variable_set(:@foo){some_heavy_calculation(...)}
```

Or, if that does not look right or seems to depart from the original usage of `instance_variable_set`, then what about having a new method?

```ruby
memoize(:foo){some_heavy_calculation(...)}
```



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