Go Dependency Injection With Functions

I'm writing warg to parse my CLI apps. In some cases, I want to be able to read an environmental variable into a data structure. I started doing this and it worked fine:

func Work() {
    // ... lots of code ...
    user, exists := os.LookupEnv("USER")
    // ... lots more code ...

Then I realized I needed to test my code...

Code depending on the environment directly like this makes tests more difficult because the test results are now also dependent on whats in the environment. This is especially difficult when the tests are parallelized. What if another test removes this environmental variable while this test is testing for it?

The classic and most obvious solution is to define an interface and two implementations - one that uses the environment, and one that lets me mock the environment, then parameterize Work with that interface:

type EnvironLike interface {
	Lookup(key string) (string, bool)

type Environ struct{}

func (Environ) Lookup(key string) (string, bool) {
	return os.LookupEnv(key)

type DictEnviron map[string]string

func (d DictEnviron) Lookup(key string) (string, bool) {
	res, exists := d[key]
	return res, exists

func Work(env EnvironLike) {
    // ... lots of code ...
    user, exists := env.Lookup("USER")
    // ... lots more code ...

Now I'm injecting the environment dependency instead of depending on it directly. A test can call Work with the dummy and not flake out if something changes with the environment:

Work(DictEnviron(map[string]string{"USER": "me"}))

And application code can depend on the environment wrapper:


Unfortunately, the Work function has this odd EnvironLike parameter. It's more difficult to scan code using Work because I have to understand what an Environ{} even does. Before I were just looking at os.LookupEnv, which is pretty familiar because it's in Go's standard library.

Can I do better? Actually, yes!

Because I really only depend on the behavior of os.LookupEnv, I can declare a function type that matches os.LookupEnv's arguments and return values, and depend on an instance of that type only. This means I don't need to wrap os.LookupEnv in a struct to use it - I can use os.LookupEnv directly and make each call site easier to read.

type LookupFunc func(key string) (string, bool)

func DictLookup(m map[string]string) LookupFunc {
	return func(key string) (string, bool) {
		val, exists := m[key]
		return val, exists

func Work(f LookupFunc) {
    // ... lots of code ...
    user, exists := f("USER")
    // ... lots more code ...

Test code can call Work with DictLookup:

Work(DictLookup(map[string]string{"USER": "me"}))

And app code can use os.LookupEnv:


Less code, and it's easier to read! Win!

See a running example in ./lookup.go or the Go Playground.