Issue #17316 has been updated by marksiemers (Mark Siemers). I agree with Sebastian that an operator is the best way to keep this code elegant. My proposal is to: **Introduce a new operator `@||` that does instance variable aware memoization succinctly** ```ruby # Allow this syntax def result @result @||= expensive_calculation end # As functionally equivalent to this def result if instance_variable_defined?(:@result) @result else @result = expensive_calculation end end ``` Reasons for this suggestion: 1. `@|` is already not valid syntax, so there should not be any issue with breaking legacy code 2. Use of the `@` symbol is a strong indicator that this impacts and is useful for instance variables 3. Keeping the `||` as part of the operator is familiar syntax for memoization 4. It is succinct, keeps things on one line, adds only one character, does not require blocks or method calls on separate lines (e.g. `memoize(:result)`) 5. If searching for memoization in the codebase - searching for `||=` will still work --- A big motivation for this comes from a recent change to rubocop-rails, which is now enforcing "Rails/FindByOrAssignmentMemoization" (see: https://rails.rubystyle.guide/\#find-by-memoization) A more rails-specific example below of this proposal: ```ruby # Previously allowed (though not performant in the case of nil returned by find_by def foo @foo ||= Foo.find_by(id:) end # The new rule enforces this syntax def foo if instance_variable_defined?(:@foo) @foo else @foo = Foo.find_by(id:) end end ``` Here's my main proposal with some reasons behind it: ```ruby def foo @foo @||= Foo.find_by(id:) end ``` --- Here are some other ideas, but I don't think any of them are as good as `@||`: ```ruby @result |||= expensive_calculation @result -||= expensive_calculation @result +||= expensive_calculation @result _||= expensive_calculation @result &||= expensive_calculation @result %||= expensive_calculation ``` ---------------------------------------- Feature #17316: On memoization https://bugs.ruby-lang.org/issues/17316#change-114252 * 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(...)} ``` -- https://bugs.ruby-lang.org/