> One of go's strengths is how easy it is to refactor

Give me a minute to collect my jaw of the floor here. Nope ... need some more time. Unless you mean in the same way as C and pascal are easy to refactor, I disagree in the strongest possible way. I may conceed to a very limited extent. While Go and it's tools don't allow refactoring, due the static nature of Go, it's actually possible, through careful design and constantly putting extra effort in, to make sure that it's reasonably easy to refactor. As long as you stay away from using interfaces, use long and unique enough names for your variables, make sure no variable names are substrings of other variable names, have a convention for polymorphic method names (ie. Matrix.MakeWithFloatArray(), Matrix.MakeWithIntArray(), Matrix.MakeWithZeroes(), ...), ...

> Implicit interfaces means that you can change a function to take an interface, and the caller who is passing in a concrete type doesn't have to get updated at all.

Yes because that's what refactoring is ... what you're showing here is called "polymorphism", and Go "doesn't support it" (except when it does, like as you point out here, in interfaces, oh and in range, make, new, append, close, copy, delete, imag, len, print, println, real, go, defer, most of which are also generic and polymorphic in really, really bad ways (some have completely unrelated and surprising behavior when passing different types to them), and I doubt I've got all of them).

> also, there's a relatively recent tool created called gorename that does 100% type-safe renaming.
> Plus there's been gofmt and gofix for forever which you can use to automatically rewrite your code.
>Finally, because almost all go code is formatted with gofmt, you can often do simple find and replace changes because all the code is completely regular.

I have tried that tool. It only does a single file. Again that makes it not refactoring. Just so we're clear. Here's the definition of refactoring :

Code refactoring is the process of restructuring existing computer code – changing the factoring – without changing its external behavior.

Which is not what those tools do. Change the name of a method ... boom 5 objects don't satisfy the interface they did 5 seconds ago anymore. Change the name of an interface ... doesn't change in all other parts of the code. Change an exported variable ... everything fails to compile.

Next major point of criticism of the go tools. When do you want to do refactoring ? Well, during development. Of course in order to refactor during development, when 2-3 of your program's files don't compile, you obviously cannot use a normal compiler to refactor, since it won't understand the program. While this is not technically part of the definition, it frustrated me to no end the first, and last, time I used gofix to attempt to refactor something. Me and vim are faster at refactoring a 10000 line Go codebases than gofix + cleaning up after it is. Gofix knows a cute trick with symbol tables that is 1% complete (because making it functional will require a full rework of the Go compiler), which is not refactoring (since it doesn't look at the full source tree), and it will require a rework of Go itself (I'm not yet positive, but I think that because Go works with implicit interfaces, it is not actually possible to refactor anything related to object methods or interfaces correctly).