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Feature #13820

Add a nil coalescing operator

Added by williamn (William Newbery) almost 3 years ago. Updated 3 months ago.

Status:
Open
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:82399]

Description

It would be nice if Ruby had an operator that only considered nil as false, like the null coalescing operators or "Logical Defined-Or operator" (Perl) found in some other languages. Ive seen things like // and //=m ?? and ??=, or ?: used for this.

This would work like || and ||= for short circuiting etc. except that only nil is considered a false condition.

While Ruby considers only "false" and "nil" as false, with everything else true ("", [], {}, etc.) I still find occasionally people trip up when using logical or, || and ||= when the value may be false.

a = 0     || 55 # = 0 Ruby already considers 0, "", etc. as true (oter languages do differ a lot here)
a = 0     ?? 55 # = 0 So no change here
a = nil   || 55 # = 55, nil is false so right side is evaulated.
a = nil   ?? 55 # = 55, again no change
a = false || 55 # = 55, however false is false for logical or
a = false ?? 55 # = false, but its still a non-nil value

For example when doing things like:

def lazy
  @lazy ||= compute_this
end


def fetch(id, **opts)
  host  = opts[:host] || default_host
  https = opts[:https] || true
  port  = opts[:port] || (https ? 443 : 80)
  ...

Normally the intention is to use a default value or compute an action if no value is provided, which if the value may be false then requires special handling, or sometimes is missed and results in a bug.

Updated by shevegen (Robert A. Heiler) almost 3 years ago

I am not sure that using a special-purpose operator would make a lot of sense.

I myself use nil primarily as means to indicate a default, "non-set" value. The
moment it is set to a boolean, be it false or true, is for me an indication
that it has been "upgraded" (or set by a user on the commandline etc...)

I do also tend to explicitely query for .nil? on some objects.

By the way, did you actually propose an actual syntax? The two '?'?

I do not think that ?? has any realistic chance for implementation due to
? already being used in ruby - in method definitions or ternary
operator for example. People may wonder why there are so many ? coming out
of nowhere. (For the record, I also consider || to be not pretty ... I
strangely end up using a more verbose but explicit way to set or ensure
defaults in ruby code. I would never write a line such as "
port = opts[:port] || (https ? 443 : 80)" simply because it takes my
brain too long to process what is going on there; my code always ends
up being so simple that I do not have to think about it much at all).

Updated by phluid61 (Matthew Kerwin) almost 3 years ago

In perl I find $x // $y useful vs $x || $y because sometimes you want to accept "" and 0 as values.

In ruby the only 'defined' falsey value is false, so I'm not sure how useful this feature is here.

If you're pulling options from a hash, for example, it's probably better to use a signal like h.fetch 'x', y to show that you accept falsey values from the hash, and/or x.nil? ? y : x to show that you explicitly only don't want nil

Updated by williamn (William Newbery) almost 3 years ago

shevegen (Robert A. Heiler) wrote:

By the way, did you actually propose an actual syntax? The two '?'?

Not really personally set on any given syntax, just ?? and // are familiar to me from other programming. Although actually for ?? specifically, I guess the fact Ruby uses it in both methods and ternary causes a conflict rather than just one or the other (x.nil?? "was nil" : "not nil"). I wouldn't know if the parser can figure that out or not.

But more the concept that any specific syntax.

I myself use nil primarily as means to indicate a default, "non-set" value. The
moment it is set to a boolean, be it false or true, is for me an indication
that it has been "upgraded" (or set by a user on the commandline etc...)

Hmm, maybe I didn't explain clearly. That is pretty much the pattern I come across repeatedly in Ruby code, and it fails for the false value because false is not "upgraded" when people do "x || my_default".

"a truthy value" || foo("something else") # The operator also short circuits so the method `foo` will never even get called
false || foo("something else") # Left side is falsy, so evaluate the right side, but is often not the intent
nil || foo("something else") # Nil is also falsy, so evaluate the right side

"a truthy value" ?? foo("something else") # String is true and not nil, so nothing changes here
false ?? foo("something else") # This changes. Left side is not nil, so the right side is never evaluated
nil ?? foo("something else") # Like with `||`, left side is nil so evaluate the right side

# so this "does the right thing", as far as my maybe not great example goes
https = opts[:https] ?? true

phluid61 (Matthew Kerwin) wrote:

In perl I find $x // $y useful vs $x || $y because sometimes you want to accept "" and 0 as values.
But not false?

While hash.fetch is nice, I still see || used a lot, in places that maybe wont convert so nice. Also it wont short circuit, in the event the default is not trivial (e.g. with say active record stuff, its easy to have something that goes to the DB without really thinking about it).

opts[:foo] || @foo_config || App.config.foo # Occasionally I see 3 or more chained together
hash[:foo] ||= fetch_foo # fetch doesn't assign the value like this does, and it wont short circuit `fetch_foo`
@lazy ||= calc_lazy # Sometimes used with non-hashes

But yes your right, they can all be done other ways, and maybe the better answer is to discourage || in the first place, but I struggled finding ones as tidy to suggest instead.

opts.fetch(:foo, !@foo_config.nil? @foo_config : App.config.foo)
hash[:foo] = fetch_foo if !hash.has_key?(:foo)

@lazy = calc_lazy if @lazy.nil?
@lazy # needed because get nil if the if condition is false

if @lazy.nil?
  @lazy = calc_lazy
end
@lazy

Updated by phluid61 (Matthew Kerwin) almost 3 years ago

williamn (William Newbery) wrote:

phluid61 (Matthew Kerwin) wrote:

In perl I find $x // $y useful vs $x || $y because sometimes you want to accept "" and 0 as values.

But not false?

Not in perl ;)

While hash.fetch is nice, I still see || used a lot, in places that maybe wont convert so nice. Also it wont short circuit, in the event the default is not trivial (e.g. with say active record stuff, its easy to have something that goes to the DB without really thinking about it).

Yes, short-circuit is handy. It's why I was a proponent of &.. Maybe it's okay to add // even if it's only used sometimes.

Updated by nobu (Nobuyoshi Nakada) almost 3 years ago

williamn (William Newbery) wrote:

shevegen (Robert A. Heiler) wrote:

By the way, did you actually propose an actual syntax? The two '?'?

Not really personally set on any given syntax, just ?? and // are familiar to me from other programming. Although actually for ?? specifically, I guess the fact Ruby uses it in both methods and ternary causes a conflict rather than just one or the other (x.nil?? "was nil" : "not nil"). I wouldn't know if the parser can figure that out or not.

?? is a string literal, and // is a regexp literal.

def fetch(id, **opts)
  host  = opts[:host] || default_host
  https = opts[:https] || true
  port  = opts[:port] || (https ? 443 : 80)

Why not keyword arguments?

#6

Updated by swrobel (Stefan Wrobel) 4 months ago

  • Subject changed from Add a nill coalescing operator to Add a nil coalescing operator

Updated by bsarrazin (Ben Sarrazin) 4 months ago

Kotlin has this feature, Swift has this feature, many other languages have this feature.
Ruby needs this feature :D

"a truthy value" || foo("something else") # The operator also short circuits so the method `foo` will never even get called
false || foo("something else") # Left side is falsy, so evaluate the right side, but is often not the intent
nil || foo("something else") # Nil is also falsy, so evaluate the right side

"a truthy value" ?? foo("something else") # String is true and not nil, so nothing changes here
false ?? foo("something else") # This changes. Left side is not nil, so the right side is never evaluated
nil ?? foo("something else") # Like with `||`, left side is nil so evaluate the right side

# so this "does the right thing", as far as my maybe not great example goes
https = opts[:https] ?? true

This is exactly why Ruby needs this feature. The lack of type safety, combined with the fact that many tools are taking "false" string as input is compelling argument to add this to the language.

Updated by shyouhei (Shyouhei Urabe) 3 months ago

bsarrazin (Ben Sarrazin) wrote in #note-7:

Kotlin has this feature, Swift has this feature, many other languages have this feature.

No. Kotlin does not have this feature (distinguish false and null). It is a really bad idea for you to refer Kotlin to have something like that. Kotlin is a statically typed language, and its || operator does not take nullable values. No confusion over false versus null must happen. That is why they need ?: operator; they need something similar to || which also work for nullables.

So if you want a ruby operator because false and nil are confusing, that's a totally different story than Kotlin's.

PS. I'm not against the feature itself. I'm just telling that other languages have their own design that do not immediately apply here.

Updated by sawa (Tsuyoshi Sawada) 3 months ago

Your proposal to distinguish nil from false is ad hoc, and is not a real solution for your use case, which is to add a value to a hash only when it does not yet have a corresponding key.

Following your way of doing it, you would still not be able to distinguish opts1 that explicitly has a key-value pair :foo => nil and opts2 that lacks such pair.

opts1 = {foo: nil}
opts2 = {}

opts1[:foo] # => nil
opts2[:foo] # => nil

Particularly, applying your code to update opts1 would overwrite the explicit nil value (?? stands for your proposed feature):

opts1[:foo] ??= "foo"

opts1[:foo] # => "foo"

Ruby is aware of such use case, and has already prepared a real solution: the Hash#key? method. The correct way of doing it is:

opts1[:foo] = "foo" unless opts1.key?(:foo)

Another way of doing it is:

opts1 = {foo: "foo"}.merge(opts1)

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