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What: debug-level logging that vanishes from production code

Go Reference

How to import the package

import ""

(Do not use the direct path to the repo.)

What does what do

what is a set of simple and easy logging functions, suitable for tracing any kind of activities in your code. what can print the current function name, quickly Printf-format your data, and dumps data structures.

And last not least, no what calls reach your production binary (unless you want it so). Debug-level logging is for developers only.

Who need this?

You definitely should give what a closer look if you -

How does it work?

First of all, what is intended for debug-level logging only. So,

  • Use what for tracing and debugging your code. ("Does my code do what I intended? Does this variable contain what I expect? Why does the loop not stop when the break condition should be fulfilled?...")
  • Use log for user-facing log output. ("What was the app doing before it said, 'cannot connect to server'? Did that service already sync or is it still waiting for other services?...")

You have to explicitly enable what logging through build flags (see below).

Available functions

what.Happens("Foo: %s", bar) // log.Printf("Foo: %s\n", bar)
what.If(cond, "Foo: %s", bar) // only print if cond is true
what.Func() // Print out the fully qualified function name
what.Is(var) // dump the structure and contents of var 

Spread these calls across your code, especially in places you want to observe closer.

Debug-level logging is useful alongside unit testing as well as using a debugger. It does not attempt to replace any of these concepts.

Enabling and disabling

Enable all functions

Simply pass the what tag to go build, go install, go test etc:

go build -tags what

And now just lean back and see your code talking about what it does.

Enable specific functions

To reduce the noise, you can decide to compile only specific parts of what:

  • whathappens only enables what.Happens() and what.If().
  • whatfunc only enables what.Func().
  • whatis only enables what.Is().

All disabled functions get replaced by no-ops.


go build -tags whathappens

You can also choose a combination of the above, for example: go build -tags whathappens,whatis

Enable debug logging for specific packages only

Go's build tag mechanism cannot help here, so this is done through an environment variable called "WHAT".

To enable specific packages for debug logging, set WHAT to a package name, or a list of package names.

Disable what

Nothing easier than that! Without any of the above build tags, all funtions get replace by no-ops, ready for being optimized away entirely (if the compiler decides to do so).

  • No log output
  • No bloated binary
  • No security leak from chatty binaries.


  • Uses only stdlib log, no custom logger configurable
  • No custom variable dumper/pretty-printer. At the moment, what uses See Spew's docs about the syntax used for printing a variable.


Although go run should recognize all build flags that go build recognizes (including -tags), it seems that go run main.go -tags what does not consider the tag. Use go build -tags what && ./main instead.


Debug-level logging for developers (only!)








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