Home Golang - Optional Arguments for APIs

Golang - Optional Arguments for APIs

I was recently directed towards Dave Cheney’s article Functional options for friendly APIs where he shares his thoughts on designs for optional parameters in APIs. Dave ends with a proposal for functional arguments that are optionally passed to a type constructor. There is no question this design is superior to having a single constructor with lots of arguments.


Dave’s design is overkill for 99% of use cases and imposes an unnecessary tax on both the maintainer and the consumer of these APIs.

Developers integrating with these APIs are consumers, so are readers (aka. code reviewers).

My proposal

A simpler alternative: two constructors, one is default, the other accepts a config struct.

Here is my proposed design for Dave’s constructors in term:

package term

// I identified just three options after a quick scan of the README:
// Baud rate, and either CBreakMode or RawMode.
type Options struct {
    CBreakMode bool  // Defaults to RawMode if false
    Baud               int

func Default(name string) (*Term, error) {...}

func Custom(name string, options Options) (*Term, error) {...}

What we gain…

In terms of usage

Decreased verbosity: occurrences of the symbol term is decreased. The magnitude of this benefit increases linearly with the number of optional parameters:

package consumer

import "github.com/pkg/term"

func DaveDesign() {
    // default
    term, err := term.Open("/dev/ttyUSB0")

    // custom
    term, err := term.Open(

func MyDesign() {
    // A ctor named 'Default' immediately conveys the possibility of
    // customization to a consumer
    term, err := term.Default("/dev/ttyUSB0")

    // custom
    term, err := term.Custom(
            Baud:               57600,
            CBreakMode: true,

In terms of maintenance

Decreased number of unit tests: reducing the set of options to a value object renders tests for them needless.

What we lose…

In terms of usage

Nothing as far as I can see.

The symbol Default clearly signals the possibility of custom Terms such that a developer consuming this API would seek out alternatives if required. This means this design has no added confusing aspects.

In terms of maintenance

N/A. We improve maintainability by reducing the number of artifacts we need to test.

Any validations and/or computations can be extracted unto their own functions (whether static or member functions) of the constructor’s type.

This post is licensed under CC BY 4.0 by the author.

Variance in Java

OAuth2 Bearer Token Usage