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Feature #8053

Make coercion if #=== operator doesn't know what to do

Added by prijutme4ty (Ilya Vorontsov) about 6 years ago. Updated almost 6 years ago.

Status:
Rejected
Priority:
Normal
Target version:
[ruby-core:53245]

Description

Related to http://bugs.ruby-lang.org/issues/7604
It'd be fine if === will do coerce when it doesn't know what to do. In ticket above I gave use-case for case-statement which needs that any ruby object could be coerced to a "pattern".
For example

case arr.end_with?
when ['several', 'words'] then puts 'arr have last two elements: several, words'
when ['word'] then puts 'arr have last element: word'
else puts 'cannot understand'
end

It'd be possible if Array#===(predicate) will simply use usual coerce call: predicate.coerce(arr) -- Predicate#coerce is a good point to understand behavior of such matching.

Another example:

class FalseYielder
def ===(other)
false
end
end
class MyString
def initalize(string); @string = string; end
attr_reader :string
def coerce(other)
if other.is_a? Regexp
[other, self.string]
else
[FalseYielder.new, self]
end
end
end

case MyString.new('abcpatdef')
when /pat/ then 'it works'
end

Here we shouldn't monkeypatch Regexp class and it's good, we can make an error only in our own class MyString.

History

Updated by zzak (Zachary Scott) almost 6 years ago

  • Category set to core
  • Status changed from Open to Assigned
  • Assignee set to matz (Yukihiro Matsumoto)
  • Target version set to 2.6

Updated by matz (Yukihiro Matsumoto) almost 6 years ago

  • Status changed from Assigned to Rejected

Objects except for numbers does not have coerce protocol. I am not sure how far OP wants to dig in.
Design and implement coerce protocol? Or special kind of coercion for case statement?
In that sense, this proposal is half-baked.

Re-submit if you come up with concrete behavior definition, e.g. what "doesn't know" mean.

Matz.

Updated by Anonymous almost 6 years ago

Sorry, Ilya, I already did some thinking, but didn't finish that flex-coerce yet.

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