use `raise_error` matcher in rspec

Yesterday I came accross a tricky useage of raise_error matcher in rspec, first I try to use the one line syntax is_expected.to

it { is_expected.to raise_error(SomeError) }

but I got an eror which stops the test before the test is finished.

It is confusing in the first time, because is_expected.to is shortcut for expect(subject).

Then I found this excellent explanation in Github isses.

So expect(subject) is not a block, subject is executed instantly, so rspec will stop before evaluating raise_error matcher.

In order to make it work, we can make subject itself a block

subject { -> { raise SomeError } }

it { is_expected.to raise_error(SomeError) }

But this syntax is creepy, instead it’s better to use

subject { raise SomeError }

expect { subject }.to raise_error(SomeError)