Ezenv Golang

TLDR

Ezenv uses golang generics to remove boilerplate surrounding retrieving ENV vars when initalising applications.

You specify a custom type, eg type CorsAllowedOrigins []string and it will map the semi-colon delimited env var CORS_ALLOWED_ORIGINS into that variable, throwing a fatal if the env var isn't set.

It can be used in the context of Dependency Injection, or without DependencyInjection.

Use case + example

Oftentimes, when using frameworks such as Uber Fx, you'll provision your services at the top level, and then use dependency injection to wire the services together.

You'll probably have written code like this:

// service.go

type CorsAllowedOrigins []string
type Port string

...

func NewHttpService(corsAllowedOrigins CorsAllowedOrigins, port Port) {
   ...
}

-----
// main.go

func GetPort() Port {
    port := os.Getenv("PORT")
    if port == "" {
        log.Fatalln("PORT env var not set.")
    }
    return port
}

func GetAllowedOrigins() CorsAllowedOrigins {
    allowedOriginsString := os.Getenv("CORS_ALLOWED_ORIGINS")
    if allowedOriginsString == "" {
        log.Fatalln("CORS_ALLOWED_ORIGINS env var not set.")
    }
    return strings.Split(allowedOriginsString, ";")
}

app := fx.New(
    fx.Provide(GetPort),
    fx.Provide(GetAllowedOrigins),
    fx.Provide(NewHttpService),
)
app.Run()

The first function GetAllowedOrigins is boilerplate. Ezenv removes the need for this boilerplate.

The above code becomes:

// service.go

type CorsAllowedOrigins []string
type Port string

...

func NewHttpService(corsAllowedOrigins CorsAllowedOrigins, port Port) {
   ...
}

-----
// main.go

app := fx.New(
    fx.Provide(ezenv.Provider[Port]),
    fx.Provide(ezenv.SliceProvider[CorsAllowedOrigins]),
    fx.Provide(NewHttpService),
)
app.Run()

How it works

The function ezenv.SliceProvider[CorsAllowedOrigins]() returns a provider, which is a function that returns a function which has the specified type as a return value.

When it is passed CorsAllowedOrigins, it will look for an env var named CORS_ALLOWED_ORIGINS. This is "magic", and so it's important to remember to have naming consistency.

ezenv.Provider[T]

Return a custom type that is a builtin data type.

ezenv.SliceProvider[T]

Returns a custom type that is a slice of builtin data types.

Supported types

Currently has support for:

Builtins

int, int32, int64
string

Slices

[]int, []int32, []int64
[]string

When not using dependency injection

func TestParseStringArrayEnvVar(t *testing.T) {   
    os.Setenv("STRING_LIST", "Alice;Bob;Charlie")   

    parts := ezenv.SliceProvider[StringList]()

    if parts[0] != "Alice" || parts[1] != "Bob" || parts[2] != "Charlie" {   
        t.Error("parts slice elements should be {1, 2, 3}")   
    }
}