Feature #17316: On memoization

• Status: Open
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

I have seen so many attempts to memoize a value in the form:

@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:

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?

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?

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

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[marcandre (Marc-Andre Lafortune)]

Memoization is tricky, not just for nil/false values. What about freezing that object? What about calling Ractor.make_shareable on it?

I just released a small gem to deal with memoization that:

• works with nil/false results.
• works for methods accepting arguments
• works for frozen objects
• is Ractor-ready in that the object can be made Ractor-shareable.

Gem is here: https://github.com/marcandre/ractor-cache
Comments welcome :-)

I think more strategies might be useful, for example accessing the cache via a Ractor/SharedHash, but haven't implemented that.

[sawa (Tsuyoshi Sawada)]

marcandre (Marc-Andre Lafortune) wrote in #note-3:

I just released a small gem to deal with memoization

Looks interesting.

[marcandre (Marc-Andre Lafortune)]

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?

I would like Kernel#instance_variable_get (not _set) to accept a block like Hash#fetch for when the instance variable is not set.

[sawa (Tsuyoshi Sawada)]

marcandre (Marc-Andre Lafortune) wrote in #note-5:

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?

I would like Kernel#instance_variable_get (not _set) to accept a block like Hash#fetch for when the instance variable is not set.

That also makes sense. Either is fine with me.

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[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.

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()

[sebyx07 (Sebastian Buza)]

IMO there should be an operator in the language directly to keep it more dry.

def my_method # current implementation
  return @cache if defined? @cache
  @cache = some_heavy_calculation
end

def my_new_method
  @cache ?= some_heavy_calculation
end

[marcandre (Marc-Andre Lafortune)]

Dan0042 (Daniel DeLorme) wrote in #note-8:

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).

Indeed. I refactored it to use Ractor.current[] and a WeakMap. I removed the other ways as I can't think of a case where this isn't the best way to go.

[[e-mail] (Joel Drapper)]

I've been experimenting with doing memoization by passing a block to attr_reader / attr_accessor, e.g.

attr_reader(:foo) { something_slow }

or

attr_reader :foo do
  something_slow
end

I prototyped this in Ruby to get a feel for what it's like to use. https://gist.github.com/joeldrapper/7e35f2f5f906344195c121801ddd28d4

[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

# 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:

# 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 is the proposal in the context of rails find_by

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 @||:

@result |||= expensive_calculation
@result -||= expensive_calculation
@result +||= expensive_calculation
@result _||= expensive_calculation
@result &||= expensive_calculation
@result %||= expensive_calculation

[Dan0042 (Daniel DeLorme)]

marksiemers (Mark Siemers) wrote in #note-12:

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:

# 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

This is appalling. If "best practices" are going to encourage this kind of un-ruby-ish horror, this memoization issue is more urgent to solve than I had expected.

Personally I prefer a DSL like memo def foo but this @||= idea is pretty interesting in how it communicates this is a syntax that applies to instance variables.

[ixti (Alexey Zapparov)]

def foo
  if instance_variable_defined?(:@foo)
    @foo
  else
    @foo = Foo.find_by(id:)
  end
end

This is appalling. If "best practices" are going to encourage this kind of un-ruby-ish horror, this memoization issue is more urgent to solve than I had expected.

Firstly, rbocop is totally optional. Secondly, even rubocop is suggesting something more elegant than the above example:

def current_user
  return @current_user if defined?(@current_user)

  @current_user = User.find_by(id: session[:user_id])
end

Which is, in my humble opinion, pretty common practice for memoizing falsey values in general.

Personally I prefer a DSL like memo def foo but this @||= idea is pretty interesting in how it communicates this is a syntax that applies to instance variables.

Neither DSL nor proposed syntax are good IMO as they add a lot of cognitive burden. Although, I think DSL is still better if anything.

[Dan0042 (Daniel DeLorme)]

ixti (Alexey Zapparov) wrote in #note-14:

Firstly, rbocop is totally optional. Secondly, even rubocop is suggesting something more elegant than the above example:

Ah yes you're right. I read "The new rule enforces this syntax" and took it at face value. But yeah the link has the much more common idiom return @foo if defined?(@foo)
Also I got the impression this was an official style guide from the Rails team, but now I'm not so sure.
Sorry for the overreaction.

[matz (Yukihiro Matsumoto)]

I understand the motivation, but the proposed "@||=" is unacceptable. Other suggestions were made, but I don't think any of them adequately express the intent.
If there are no good operator name suggestions in the future, it would be better to explicitly use defined?.

Matz.

[matheusrich (Matheus Richard)]

@matz (Yukihiro Matsumoto) I propose ?= as the uninitialized assignment operator:

@foo ?= some_heavy_calculation(...)

[matz (Yukihiro Matsumoto)]

Currently, ?= means "a single character =". We don't want to break current available syntax, unless absolutely necessary.

Matz.

[Dan0042 (Daniel DeLorme)]

The single-character syntax itself isn't a problem if limited to instance variables. foo ?= is already valid and shouldn't change, but @foo ?= isn't currently valid, so it could serve as "assign if ivar undefined."

The problem is consistency. All operator-equals decompose uniformly (a += b -> a = a + b) but ?= and its variations (like @||=) would be very different. What should happen with cases like:

foo      ?= expr
foo[key] ?= expr
foo.bar  ?= expr

If that's a syntax error, it's inconsistent with other operators like += or ||=.
If it means "assign if not nil", it's inconsistent with @foo ?= expr as "assign if undefined".
And if @foo ?= expr is changed to mean "assign if not nil" then we're back to the nil coalescing operator which is a different topic (#13820)

An operator for "assign if undefined" is appealing, but the implications don't work out cleanly.