Update StartWith, EndWith to use start_with?, end_with? if available #1327

bclayman-sq · 2021-10-13T15:05:09Z

This addresses issue #1025.

With this change, the StartWith matcher will rely on an object's start_with? method if available. Similarly, the EndWith matcher will rely on an object's end_with? method if available.

This is especially useful when a class implements start_with? but not the indexing operator, or end_with? but not the indexing operator.

This addresses issue rspec#1025. With this change, the StartWith matcher will rely on an object's start_with? method if available. Similarly, the EndWith matcher will rely on an object's end_with? method if available. This is especially useful when a class implements start_with? but not the indexing operator, or end_with? but not the indexing operator.

pirj

Looks great!
Wondering why it wasn't done like that from the very beginning.

Wondering if we need to check if there are multiple args:

"abc".start_with?('a', 'c') # => true

https://ruby-doc.org/core-2.7.1/String.html#method-i-start_with-3F seems to allow all prefixes, while the matcher's semantic is different.

pirj · 2021-10-14T06:44:58Z

lib/rspec/matchers/built_in/start_or_end_with.rb

@@ -73,6 +76,10 @@ def subset_matches?
        def element_matches?
          values_match?(expected, actual[0])
        end
+
+        def method


Let's call it method_name not to override Kernel#method.

Sounds good!

pirj · 2021-10-14T06:49:35Z

spec/rspec/matchers/built_in/start_and_end_with_spec.rb

-      }.to fail_with("expected #{actual.inspect} to start with 0, but it cannot be indexed using #[]")
+    context "with an object that responds to start_with?" do
+      it "relies on start_with?" do
+        my_struct = Struct.new(:foo) do


Maybe

has_built_in_start_with = double(:start_with? => true)

Another approach would be to actually check if start_with? was called:

has_start_with_built_in = double expect(has_start_with_built_in).to receive(:start_with?).with(0).and_return(true) expect(has_start_with_built_in).to start_with(0)

Ah yeah, this is much nicer than my Struct approach, thanks for pointing it out!

JonRowe

Nice work, but this can't be merged into main as its a breaking change in behaviour, so would have to be for rspec 4.

JonRowe · 2021-10-13T15:10:16Z

lib/rspec/matchers/built_in/start_or_end_with.rb

@@ -73,6 +76,10 @@ def subset_matches?
        def element_matches?
          values_match?(expected, actual[0])
        end
+
+        def method


Don't override method, which is an existing Ruby method, use an alternative like operator_name or comparison_name

JonRowe · 2021-10-13T15:11:12Z

lib/rspec/matchers/built_in/start_or_end_with.rb

@@ -34,6 +35,8 @@ def description
      private

        def match(_expected, actual)
+          # use an object's start_with? or end_with? as appropriate


See the comment on method but this should just refer to the name of the method called to determine what to call.

JonRowe · 2021-10-13T15:12:12Z

lib/rspec/matchers/built_in/start_or_end_with.rb

          super.tap do |msg|
            if @actual_does_not_have_ordered_elements
              msg << ", but it does not have ordered elements"
            elsif !actual.respond_to?(:[])
-              msg << ", but it cannot be indexed using #[]"
+              msg << response_msg


This should be reverted and a seperate part added to the conditional

Oh yeah, I botched something here 😅 . I meant it to be:

response_msg = ", but it does not respond to #{method} and cannot be indexed using #[]" super.tap do |msg| if @actual_does_not_have_ordered_elements msg << ", but it does not have ordered elements" elsif !actual.respond_to?(method) && !actual.respond_to?(:[]) msg << response_msg end

Here's what I'm thinking: Suppose actual does have ordered elements. We should only fail if actual responds to neither start_with? nor indexing. If it responds to either, we don't need to append to our failure message.

Does that sound reasonable?

Its more that there is two components to this failure, it either responds to the method and failed that check, or it fell back to the original check

Hmm, I don't think I follow, sorry. It seems to me that there are a few cases where we:

does implement start_with?, doesn't implement []

doesn't implement start_with?, does implement []

implements both

implements neither

Only 4 should produce an error message here, which is why I wrote up the combined error message. It sounds like your suggestion is to have something like:

super.tap do |msg| if @actual_does_not_have_ordered_elements msg << ", but it does not have ordered elements" elsif !actual.respond_to?(method) msg << "doesn't respond to #{method}" elsif !actual.respond_to?(:[]) msg << "cannot be indexed using #[]" end

But that doesn't seem to handle case 4 correctly? Maybe I'm missing something!

JonRowe · 2021-10-14T12:54:18Z

spec/rspec/matchers/built_in/start_and_end_with_spec.rb

+        expect {
+          expect(actual).to start_with 0
+        }.to fail_with("expected #{actual.inspect} to start with 0, but it does not respond to start_with? and cannot be indexed using #[]")
+      end


Rather than removing the old spec and nesting additional contexts, these should be seperate cases with the original left as is

Oh I'd love to hear more about the rationale for this! I often set up my tests to be mutually exclusive and nesting makes that easier for me. So they often have this shape:

when A when B when not B when not A when B when not B

I wasn't totally sure about leaving this case as is because now, simply knowing that actual responds to :[] doesn't fully specify what should happen. If actual doesn't respond to start_with?, it should have one behavior. If it does, it should have a different behavior.

Could you tell me more about how you were thinking of structuring this given the above? I'm happy to follow any convention here, mostly wanted to ask for my own understanding!

JonRowe · 2021-10-14T12:54:26Z

spec/rspec/matchers/built_in/start_and_end_with_spec.rb

+        expect {
+          expect(actual).to end_with 0
+        }.to fail_with("expected #{actual.inspect} to end with 0, but it does not respond to end_with? and cannot be indexed using #[]")
+      end


Same as above

bclayman-sq · 2021-10-14T14:15:01Z

Looks great! Wondering why it wasn't done like that from the very beginning.

Wondering if we need to check if there are multiple args:
"abc".start_with?('a', 'c') # => true
https://ruby-doc.org/core-2.7.1/String.html#method-i-start_with-3F seems to allow all prefixes, while the matcher's semantic is different.

Yeah, that's a great point. If we do this:

expect("abc").to start_with("a", "c")

Then expected is ["a", "c"] and my implementation doesn't handle this correctly: "abc".start_with?(["a", "c"]) fails with an error.

I think it's OK to assume that any class that implements start_with? allows you to pass multiple args (like in your string example). In light of that, I think I can splat expected like this:

return actual.send(method_name, *expected) if actual.respond_to?(method_name)

As you and @JonRowe pointed out, this is a breaking change, so totally get that this would have to be in the 4.0 release 👍

pirj · 2021-10-15T12:09:06Z

I'm not convinced to make a breaking change.
There is be_start_with dynamic matcher that works similarly to start_with?. And we can keep the known behaviour for start_with intact.

To still be able to benefit from this change, I'd suggest to only delegate to start_with? if the matcher gets a single argument. For multiple - fallback to the previous behaviour.

On a side note - adding deprecation messages is not much fun.

bclayman-sq force-pushed the bclayman/update-start-end-with branch from 41107b7 to fdc2d26 Compare October 13, 2021 15:36

pirj reviewed Oct 14, 2021

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Update StartWith, EndWith to use start_with?, end_with? if available #1327

Update StartWith, EndWith to use start_with?, end_with? if available #1327

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pirj commented Oct 15, 2021

Update StartWith, EndWith to use start_with?, end_with? if available #1327

Are you sure you want to change the base?

Update StartWith, EndWith to use start_with?, end_with? if available #1327

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