I’ve been using mock/Gomock to write tests in my personal project. When you’re building something in a new language, it is hard to prioritize learning every tool in your toolchain. For me, I’ve been writing custom and suboptimal code for Gomock because of a nifty but undocumented API call .Do.

In many cases, I wan to match subsets of a complex object while ignoring irrelevant parts. e.g., verify a function is invoked with a list of User objects, but only verifying the email addresses. To do that in a generic way, I wrote a custom Matcher API that uses text/template to describe what parts of the object to match. Thus, my mock-and-verify code looks like: