ohir / condprint

Conditional Printf for building log messages.

condprint

import "github.com/ohir/condprint"

Package condprint provides a fmt.Printf type function writing to either os.Stdout or to any other io.StringWriter. This function is instantiated via the Verbose guard function taking a boolean flag V and up to three optional parameters:

• target io.StringWriter. Returned printf will write to it. Usually an address of local strings.Builder is given as a target, otherwise the default os.Stdout is used.
• prefix string, to be prepended as-is to the output
• prefix func() string returning string to be prepended to the output Both prefix string and prefix function can be given, and their precedence on input parameters list will be preserved in the output.

If V is false at Verbose call site, the noop empty function is returned instead.

The returned from Verbose printf function has two additional methods: If and IfNot, that print according to condition in place (and as method names tell). Eg.

  p := condprint.Verbose(loglevel > 3, &logsink, tmstamp) // function tmstamp
  p("Writes always")
  p.If(details, "Only if condition is true")
  p.IfNot(details, "Only if condition is false")

Both If and IfNot return condition intact.

func Verbose

func Verbose(V bool, opts ...interface{}) func(fmt string, a ...interface{})

function Verbose(V bool, ...opts) (printf) returns a printf type function (for V being true), or an empty stub (for V being false).

Verbose may take three optional parameters:

- prefix string, to be prepended as-is to the output
- prefix function returning string that gets prepended to the output
- target io.StringWriter printers will write to, if V is true.
Both prefix string and prefix function can be given, and its precedence
on input parameters list will be preserved in the output.

Note: to indicate options misuse early, Verbose will panic if given optional parameter of a wrong type.