Misc #16464


Which core objects should support deconstruct/deconstruct_keys?

Added by zverok (Victor Shepelev) over 3 years ago. Updated 5 months ago.



Now, when pattern matching is out, I believe it is worth discussing which core and standard library objects should be matchable "out of the box".

My proposals, as of now, are:

1. Object#deconstruct, returning [self].


# this works:
1 in Integer
# this works:
1 in ..0
# this does NOT:
1 in Integer(..0)
# NoMatchingPatternError (1)

I believe the latter example looks pretty logical (and can be used in some flexible methods like "if it is a positive integer, it is index in the array, if it is negative integer, it is backward index, and if it is float, it should be calculated as a mean of nearby elements")

2. Time#deconstruct_keys

Justification is obvious:

case created_at
when year: 2019, month: 11..12 => m, day:
  p "Created at #{day}.#{m} this year"
  # ...

(Probably the same for Date and DateTime)

3. Set#deconstruct

Seems "logical" as set is a sequence, but I can't think of a good realistic example :)

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

I think when possible every object should support deconstructing IF
it makes sense. If it walks like a duck, talks like a duck, then it
can be dequacked (deconstructed like a duck).

I guess one question may be how useful something it is, whether there
is an actual use case or not - people to use that. Perhaps it may be
better to see and wait for (several) people who really had a use
case to do so and evaluate again in a few months.

1 in Integer(..0)

"I believe the latter example looks pretty logical "

To me this looks very, very, very strange.

Is that still ruby at all? :P

I guess it follows from a logical continuation, e. g. " if
x in y" works, and "beginless ranges" work, then the above
should work too. But the syntax is so strange - I wonder
if I am the only one feeling about that so if that is the
case I'll happily quiet down. But to my eyes it looks very

Actions #2

Updated by Dan0042 (Daniel DeLorme) over 3 years ago

1 in Integer(..0)

"I believe the latter example looks pretty logical "

To me this looks very, very, very strange.

Same here. It looks like trying to convert a beginless range to an Integer. I understand how Object#deconstruct would work with the array pattern to allow this, but it feels a bit hacky. An object with deconstruct should have some kind of enumerable/tuple-like quality such that it makes sense to represent it as an array.

Given that "Alternative pattern" is represented as pat | pat | ..., using & like this would look more intuitive to me:

 1 in Integer & ..0

Time#deconstruct_keys is pretty obvious indeed.

Set#deconstruct might make sense but I think that a Set is usually considered un-ordered? But an array pattern matches in order with the values provided by deconstruct, so Set[1,2,3] in Set[3,2,1] would be false. Quite a gotcha, no?

Updated by decuplet (Nikita Shilnikov) over 3 years ago

These probably should be filed as separate feature requests. I'm already using PM in production and the idea about deconstructing Time instances also came into my mind. Object#deconstruct is a lot more subtle, I wouldn't say it's that obvious, I'd rather have it for some built-in classes such as Time or Integer.

Actions #4

Updated by zverok (Victor Shepelev) 5 months ago

  • Status changed from Open to Closed

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