Option builder generator.


go get github.com/sirkon/[email protected]


There are so-called functional options described nicely in this article.

But imagine a situation where we have a custom buffered io package where both reader and writer need a way to set up a buffer size and there will be also options that are reader and writer specific. Something like:

r, err := cio.NewBufferedReader(
	cio.WithMaxReadPosition(readend), // This is reader specific options. 


w, err := cio.NewBufferedWriter(w, cio.WithBufferSize(size))

This code will not compile because options for reader and writer will have different type in order to prevent cio.WithMaxReadPosition usage for writer.

There are ways to overcome this using generics: article

But in the end the usage will be not exactly nice – shared options to have mandatory type parameter:

r, err := cio.NewBufferedReader(r, cio.WithBufferSize[*BufferedReader](size))


This utility provides a different way to functional options: we don’t set options directly, we fulfill them with a generated builder:

r, err := cio.NewBufferedReader(r, cio.BufferedReaderOptions().BufferSize(size))

where builders for reader and writer will have different types and therefore no type clashes.

How to use.

Option builder generation requires a package with Go code where you set up options for each type.

Imagine we have github.com/company/cio repository with BufferedReader type in it. Then we create a package github.com/company/cio/internal/options with a set of files in it.

// github.com/company/cio/internal/options/buffered_reader_options.go
package options

import (

// BufferedReaderBufferSize sets a buffer size, a default value is 4096.
const BufferedReaderBufferSize int = 4096

// BufferedReaderLogger sets an error logger for a buffered reader.
func BufferedReaderLogger(err error) {

Now, generate a builder:

opgen -s internal/options -d options_gen.go BufferedReader

It will look for BufferedReaderXXX named constants (will provide a default value), variables (no default value) and functions (will provide a default value – the function itself) where a type of each option will match a type of constant/variable/function.

The builder will have methods in case of the file above:

func (b BufferedReaderOptionsType) BufferSize(v int) BufferedReaderOptionsType { … }
func (b BufferedReaderOptionsType) Logger(v func(error)) BufferedReaderOptionsType { … }

And the usage is supposed to be like this:

v, err := cio.NewBufferedReader(
        Logger(func (err error) {



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