formatter

var Decamel = &Formatter{DoFmt: str.Decamel}

Decamel is preimplemented and default formatter to produce human readable error strings from function names.

func CopyFile(..)  -> "copy file: file not exists"
                       ^-------^ -> generated from 'func CopyFile'

var Noop = &Formatter{DoFmt: func(i string) string { return i }}

Noop is preimplemented formatter that does nothing to function name.

func CopyFile(..)  -> "CopyFile: file not exists"
                       ^------^ -> function name as it is: CopyFile