The Problem

You’re using a great open source library in your project, and then, one day, you find yourself thinking:

“Oh, wouldn’t be great if the author just added a tiny little log statement in that function?”.

Yeah, it really would, this would allow me to trace the value of that variable or try a small change.

What can you do: fork the library? Nope. Don’t do that.

Replace it!

You can temporarily replace the original module with a local copy, changing one line in your go.mod file.

Example

Here’s a minimal example of a small go.mod, the file every Go project uses to handle dependencies:

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module github.com/zmoog/foo

require (
    github.com/zmoog/bar v0.1.0
)

To replace the module github.com/zmoog/bar with a local copy located on your local disk, all you need to do is add a new line with the replace directive:

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module github.com/zmoog/foo

replace github.com/zmoog/bar => /Users/zmoog/code/projects/bar

require (
    github.com/zmoog/bar v0.1.0
)

That’s it! 🥳

Now, when you recompile or run your code, the local copy will be used with all your local changes.

Conclusion

The replace directive is a nice little tool for this use case.

Notice

Don’t ever commit these changes into your repo, they’re meant to be transient and should be thrown away after use.