[ruby-core:123219] [Ruby Feature#17316] On memoization

Issue #17316 has been updated by nobu (Nobuyoshi Nakada).


If it matters only for instance variables, a new syntax won't be needed.

```ruby
instance_variable_set_unless_defined(:@foo) do
  some_heavy_calculation(...)
end
```

or, like `Ractor.store_if_absent`:
```ruby
instance_variable_set_if_absent(:@foo) do
  some_heavy_calculation(...)
end
```

----------------------------------------
Feature #17316: On memoization
https://bugs.ruby-lang.org/issues/17316#change-114550

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
----------------------------------------
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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