Feature #16341: Proposal: Set#to_proc and Hash#to_proc

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

[Nondv (Dmitry Non)]

class Set
  def to_proc
    -> (x) { include?(x) } # or method(:include?).to_proc
  end
end

Usage:

require 'set'

banned_numbers = Set[0, 5, 7, 9]
(1..10).reject(&banned_numbers) # ===> [1, 2, 3, 4, 6, 8, 10]

UPD

also for hash:

class Hash
  def to_proc
    ->(key) { self[key] }
  end
end

dogs = ['Lucky', 'Tramp', 'Lady']
favourite_food = { 'Lucky' => 'salmon', 'Tramp' => 'pasta', 'Lady' => 'pasta' }

food_to_order = dogs.map(&favourite_food)

[Nondv (Dmitry Non)]

• Tracker changed from Bug to Feature
• Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN)

[zverok (Victor Shepelev)]

Since 2.5, Set implements #===, so you can just:

(1..10).grep_v(banned_numbers)
# => [1, 2, 3, 4, 6, 8, 10] 

which is pretty clear and probably more effective than proc conversion.

[Nondv (Dmitry Non)]

Well, to_proc allows to send objects as blocks which can be quite useful not just in case of select/reject. Also, probably, those two are used more often than grep/grep_v.

Another example from the top of my head is count:

dogs = Set[:labrador, :husky, :bullterrier, :corgi]
pets = [:parrot, :labrador, :goldfish, :husky, :labrador, :turtle]
pets.count(&:dogs) # ===> 3

[zverok (Victor Shepelev)]

Fair enough. ...well, you still can

pets.count(&dogs.:include?)

until the core team haven't reverted it :))))

(Which, for me, is more clear than value-objects-suddenly-becoming-procs, but apparently it is only me)

[Nondv (Dmitry Non)]

Well, to be fair, this change is just nice-to-have sugar. I don't expect it to become a thing.

I guess for now the best way to do that is:

pets.count { |x| dogs.include?(x) }
# or
pets.count(&dogs.method(:include?))

They both are "more clear than value-object-suddenly-becoming-procs". But having implicit conversion would be just a nice feature to make code more compact and expressive (MHO).
Clojure treats sets as functions, btw:

(def dogs #{:labrador :husky :bullterrier :corgi})

(count (filter dogs [:parrot :labrador :goldfish :husky :labrador :turtle]))

[shevegen (Robert A. Heiler)]

this change is just nice-to-have sugar. I don't expect it to
become a thing.

The ruby core team often points out that having good use cases may
help a proposal; and of course avoiding other problems such as
backwards-incompatibility or such.

Your initial comment is quite sparse, so zverok sort of got you to
explain more lateron. ;)

I am not really using ruby in a functional-centric manner nor do I
know clojure (aside from superficial glances), but to me personally
I am not completely sure if the use case has been explained. Unless
it was only syntactic sugar of course.

[Nondv (Dmitry Non)]

This is a syntactic sugar. Using & + to_proc in this case is the same (not technically, but algorithmically, I guess) as providing an explicit block something.some_method { |x| some_set.include?(x) }

I don't find it crucial in any way and, to be honest, I don't really use sets that much (I prefer using hashes directly). But this feature could make some code a tiny bit easier to read from English language perspective (I think)

[Nondv (Dmitry Non)]

Speaking of hashes, they could implement implicit proc conversion as well:

class Hash
  def to_proc
    ->(key) { self[key] }
  end
end

dogs = ['Lucky', 'Tramp', 'Lady']
favourite_food = { 'Lucky' => 'salmon', 'Tramp' => 'pasta', 'Lady' => 'pasta' }

food_to_order = dogs.map(&favourite_food)

[Nondv (Dmitry Non)]

The main problem is that implicit conversion can be confusing, especially, if it's not obvious what the resulting proc is going to do.

However, I think that hashes are being used mainly for making key-value pairs and accessing them and sets are being used for checking if something is included.
So usage of :[] and :include? seems appropriate and relatively straight-forward to me.

Of course, depending on the context. With map/reduce/count it does make sense indeed but maybe there're cases when it can make things hard to understand

[Nondv (Dmitry Non)]

• Subject changed from Proposal: Set#to_proc to Proposal: Set#to_proc and Hash#to_proc

[Nondv (Dmitry Non)]

• Description updated (diff)

[shan (Shannon Skipper)]

Nondv (Dmitry Non) wrote:

Speaking of hashes, they could implement implicit proc conversion as well:

class Hash
  def to_proc
    ->(key) { self[key] }
  end
end

dogs = ['Lucky', 'Tramp', 'Lady']
favourite_food = { 'Lucky' => 'salmon', 'Tramp' => 'pasta', 'Lady' => 'pasta' }

food_to_order = dogs.map(&favourite_food)

This already works! Hash#to_proc was added in Ruby 2.3. ruby-core:11653

I like the idea of Set#to_proc.

[Nondv (Dmitry Non)]

shan (Shannon Skipper) wrote:

This already works!

I can't believe I'm so oblivious :D