Ruby map() for a single object

Question

I am looking for a way to 'map' a single item in Ruby.

I want to call this function and pass it a block, the object will be yielded to the block, and then the result of the block will be returned to the caller. Exactly what map does, but for a single element.

The motivation is that sometimes you generate objects that are just used to construct something else. The original object is then no longer needed. It would be nice to just put the conversion into a block and eliminate the temporary.

As a contrived example, let's assume that I want to create an integer that represents the month/year combination. For today's date, the code would look something like:

day = Date.today
month_number = day.year * 100 + day.month

I'd really like it if I could do something like:

month_number = Date.today.some_function { |d| d.year * 100 + d.month }

But I don't know what 'some_function' is (or if it even exists).

If there is a more Ruby way of handling something like this, I'm all ears. I am aware of monkey patching classes, but I am looking to handle those cases that are a bit more transient.

Answer 1

instance_eval is what you're looking for.

month_number = Date.today.instance_eval { |d| d.year * 100 + d.month }

The |d| is also optional and self defaults to the object context.

This may satisfy your needs in a more compact way.

month_number = Date.today.instance_eval { year * 100 + month }