Feature #15722: `Kernel#case?`

• Status: Feedback
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

I often want to use === to match a single object on the right side against multiple objects on the left, as is used in case-constructions, just to return a truth value, and end up writing like this:

bar # => "bar"
flag1 = case bar; when "foo", "bar", "baz"; true; end # => true
flag2 = case bar; when Symbol, String; true; end # => true

I propose Kernel#case? that should work like this:

bar # => "bar"
bar.case?("foo", "bar", "baz") # => true
bar.case?("qux") # => false
bar.case?(Symbol, String) # => true
bar.case?(Array) # => false
bar.case? # => false

It is similar to Rails' in?, but it differs from it in that it uses === for comparison, not ==.

Or, alternatively, allowing Kernel#instance_of? and Kernel#kind_of? to allow multiple arguments may be a compromise.

[osyo (manga osyo)]

How about Enumerable#case? ?
Comparing with === like Enumerable#include?.

bar # => "bar"
["foo", "bar", "baz"].case? bar # => true
["qux"].case? bar               # => false
[Symbol, String].case? bar      # => true
[Array].case? bar               # => false

[sawa (Tsuyoshi Sawada)]

osyo (manga osyo) wrote:

How about Enumerable#case? ?
Comparing with === like Enumerable#include?.

bar # => "bar"
["foo", "bar", "baz"].case? bar # => true
["qux"].case? bar               # => false
[Symbol, String].case? bar      # => true
[Array].case? bar               # => false

What is to be evaluated is bar, not the objects that you put in the arrays. bar has to be the receiver. That also matches with how case-construction works. Furthermore, having an array as in your proposal requires additional array to be created, which will be immediately disposed.

[sawa (Tsuyoshi Sawada)]

If such method, if any, should be rather defined on Object, that would be fine. I was not sure whether I should have requested this for Kernel or for Object.

[shevegen (Robert A. Heiler)]

This is an interesting idea. I have not thought through all implications but not withholding
that, I like it so +1 from me. I think matz has to decide whether this may be useful. Since
I love case/when in ruby in general, any extension, even secondary ideas, seem useful to me. :)
Although I have to admit, I am not sure if I yet had a use case as suggested by sawa, but I
still think it is a good idea.

I think whether it is on Kernel or Object or Enumerable is not the primary concern (that can
be decided anyway) - if matz is too busy right now then perhaps this could be suggested for
the next developer meeting to get more feedback.

I did not know about Rails in?, but I do not like the name "in?". I have no problem with
"case?".

One possible question from the core team might be whether the use case is sufficient (for
many ruby users). I can not answer that myself, to be honest, if that question would come
up - but I still like the idea.

PS: I almost overlooked the comment by osyo; I agree, that is a different suggestion so
I was confused for a moment. The original suggestion by sawa is clearer IMO and more
appropriate (to the proposed suggestion); wording it like:

["foo", "bar", "baz"].case? bar 

would be different, and is actually a bit confusing to me. So it should best be ignored;
sawa's original description is clearer and makes more sense to me, but this is just an
aside - I only was confused for a moment when I read the second code part by osyo.

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[osyo (manga osyo)]

What about using #when? instead of #case??

bar # => "bar"
flag1 = case bar; when "foo", "bar", "baz"; true; end # => true
flag2 = case bar; when Symbol, String; true; end # => true

# Proposal by sawa
flag1 = bar.case? "foo", "bar", "baz" # => true
flag2 = bar.case? Symbol, String # => true

# Proposal by osyo
flag1 = bar.when? "foo", "bar", "baz" # => true
flag2 = bar.when? Symbol, String # => true

I think this is closer to the syntax of case when.
And what about adding a oneline when like a oneline in ?

bar # => "bar"
flag1 = case bar; when "foo", "bar", "baz"; true; end # => true
flag2 = case bar; when Symbol, String; true; end # => true

flag1 = bar when "foo", "bar", "baz" # => true
flag2 = bar when Symbol, String # => true

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Feedback

It is not obvious to me that #case? is useful. We need a real-world use-case.

• I don't like the name case?
• IMO, just use case statement

Matz.

[nobu (Nobuyoshi Nakada)]

osyo (manga osyo) wrote in #note-6:

I think this is closer to the syntax of case when.
And what about adding a oneline when like a oneline in ?

bar # => "bar"
flag1 = case bar; when "foo", "bar", "baz"; true; end # => true
flag2 = case bar; when Symbol, String; true; end # => true

flag1 = bar when "foo", "bar", "baz" # => true
flag2 = bar when Symbol, String # => true

As a case expr statement doesn't need a terminator (newline or semicolon) before when, it conflicts with the current syntax.