MarshalKeyvals doesn't work as expected about logfmt HOT 2 CLOSED

The function signature of MarshalKeyvals is as follows.

func MarshalKeyvals(keyvals ...interface{}) ([]byte, error)

That is, it accepts a variable number of interface{} arguments. It can be called in one of two ways.

Either explicitly listing a sequence of keys and values

logfmt.MarshalKeyvals("app", "myapp", "env", "prod", "error", "errormsg")

Or expanding a pre-existing slice in place, like so.

kvs := []interface{}{"app", "myapp", "env", "prod", "error", "errormsg"}
logfmt.MarshalKeyvals(kvs...)

(See also the section in the Go spec entitled Passing arguments to ... parameters.)

The code snippet you provided has two flaws that cause problems here.

First, the ... is missing in the call to MarshalKeyvals. Without that keyvals is not expanded into the variadic parameter and instead is passed as a single argument. That is valid because any value, even a slice, can be converted to an interface{} value. The result is that Keyvals receives a single element slice with the only element containing your slice of strings. Instead of receiving six strings it receives []interface{}{keyvals} or, expanding that, it receives []interface{}{[]string{"app", "myapp", "env", "prod", "error", "errormsg"}}, a slice within a slice, which is not what you intended.

Note that this is the reason that you don't get any output. MarshalKeyvals is a helper function that is a thin wrapper around Encoder.EncodeKeyvals, which documents more details about its behavior. The docs for that method are:

EncodeKeyvals writes the logfmt encoding of keyvals to the stream. Keyvals is a variadic sequence of alternating keys and values. Keys of unsupported type are skipped along with their corresponding value. Values of unsupported type or that cause a MarshalerError are replaced by their error but do not cause EncodeKeyvals to return an error. If a non-nil error is returned some key/value pairs may not have be written.

A []string is an unsupported key type, and thus it is skipped. Since it is the only element of the []interface{} Go creates to fill in the variadic parameter MarshalKeyvals returns an empty []byte.

If we fix that problem by adding the ... we discover the second problem. keyvals := []string{"app", "myapp", "env", "prod", "error", "errormsg"} is a slice of strings, and Go will not automatically convert a []string to a []interface{}. You can see the compiler error when you run that version on the Go playground. This is a commonly encountered issue and is covered in the Go FAQ entry Can I convert a []T to an []interface{}?

Changing the type of keyvals to a []interface{} resolves this problem. See this on the playground.

I hope that helps.

from logfmt.

thanks, that's clear! appreciate the guidance!

from logfmt.