# goose: Compile-Time Dependency Injection for Go

goose is a compile-time [dependency injection][] framework for Go, inspired by
[Dagger][].  It works by using Go code to specify dependencies, then
generating code to create those structures, mimicking the code that a user
might have hand-written.

[dependency injection]: https://en.wikipedia.org/wiki/Dependency_injection
[Dagger]: https://google.github.io/dagger/

## Usage Guide

### Defining Providers

The primary mechanism in goose is the **provider**: a function that can
produce a value, annotated with the special `goose:provide` directive. These
functions are otherwise ordinary Go code.

```go
package foobarbaz

type Foo int

// goose:provide

// ProvideFoo returns a Foo.
func ProvideFoo() Foo {
	return 42
}
```

Providers are always part of a **provider set**: if there is no provider set
named on the `//goose:provide` line, then the provider is added to the provider
set with the same name as the function (`ProvideFoo`, in this case).

Providers can specify dependencies with parameters:

```go
package foobarbaz

type Bar int

// goose:provide SuperSet

// ProvideBar returns a Bar: a negative Foo.
func ProvideBar(foo Foo) Bar {
	return Bar(-foo)
}
```

Providers can also return errors:

```go
package foobarbaz

import (
	"context"
	"errors"
)

type Baz int

// goose:provide SuperSet

// ProvideBaz returns a value if Bar is not zero.
func ProvideBaz(ctx context.Context, bar Bar) (Baz, error) {
	if bar == 0 {
		return 0, errors.New("cannot provide baz when bar is zero")
	}
	return Baz(bar), nil
}
```

Provider sets can import other provider sets.  To add the `ProvideFoo` set to
`SuperSet`:

```go
// goose:import SuperSet ProvideFoo
```

You can also import provider sets in another package, provided that you have a
Go import for the package:

```go
// goose:import SuperSet "example.com/some/other/pkg".OtherSet
```

A provider set reference is an optional import qualifier (either a package name
or a quoted import path, as seen above) ending with a dot, followed by the
provider set name.

### Injectors

An application wires up these providers with an **injector**: a function that
calls providers in dependency order. With goose, you write the injector's
signature, then goose generates the function's body.

An injector is declared by writing a function declaration without a body in a
file guarded by a `gooseinject` build tag. Let's say that the above providers
were defined in a package called `example.com/foobarbaz`. The following would
declare an injector to obtain a `Baz`:

```go
//+build gooseinject

package main

import (
	"context"

	"example.com/foobarbaz"
)

// goose:use foobarbaz.SuperSet

func initializeApp(ctx context.Context) (foobarbaz.Baz, error)
```

Like providers, injectors can be parameterized on inputs (which then get sent to
providers) and can return errors. Each `goose:use` directive specifies a
provider set to use in the injection. An injector can have one or more
`goose:use` directives. `goose:use` directives use the same syntax as
`goose:import` to reference provider sets.

You can generate the injector by invoking goose in the package directory:

```
goose
```

Or you can add the line `//go:generate goose` to another file in your package to
use [`go generate`]:

```
go generate
```

(Adding the line to the injection declaration file will be silently ignored by
`go generate`.)

goose will produce an implementation of the injector in a file called
`goose_gen.go` that looks something like this:

```go
// Code generated by goose. DO NOT EDIT.

//+build !gooseinject

package main

import (
	"example.com/foobarbaz"
)

func initializeApp(ctx context.Context) (foobarbaz.Baz, error) {
	foo := foobarbaz.ProvideFoo()
	bar := foobarbaz.ProvideBar(foo)
	baz, err := foobarbaz.ProvideBaz(ctx, bar)
	if err != nil {
		return 0, err
	}
	return baz, nil
}
```

As you can see, the output is very close to what a developer would write
themselves. Further, there is no dependency on goose at runtime: all of the
written code is just normal Go code, and can be used without goose.

[`go generate`]: https://blog.golang.org/generate

## Best Practices

goose is still not mature yet, but guidance that applies to Dagger generally
applies to goose as well. In particular, when thinking about how to group
providers into sets, follow the same [guidance](https://google.github.io/dagger/testing.html#organize-modules-for-testability)
as Dagger (provider sets are called modules in Dagger/Guice):

> Some [...] bindings will have reasonable alternatives, especially for
> testing, and others will not. For example, there are likely to be
> alternative bindings for a type like `AuthManager`: one for testing, others
> for different authentication/authorization protocols.
>
> But on the other hand, if the `AuthManager` interface has a method that
> returns the currently logged-in user, you might want to [export a provider of
> `User` that simply calls `CurrentUser()`]  on the `AuthManager`. That
> published binding is unlikely to ever need an alternative.
>
> Once you’ve classified your bindings into [...] bindings with reasonable
> alternatives [and] bindings without reasonable alternatives, consider
> arranging them into provider sets like this:
>
> - One [provider set] for each [...] binding with a reasonable alternative.
>   (If you are also writing the alternatives, each one gets its own [provider
>   set].) That [provider set] contains exactly one provider.
> - All [...] bindings with no reasonable alternatives go into [provider sets]
>   organized along functional lines.
> - The [provider sets] should each include the no-reasonable-alternative
>   [provider sets] that require the [...] bindings each provides.

One goose-specific practice though: create one-off types where in Java you
would use a binding annotation. For example, if you need to pass a string
through the dependency graph, you would create a wrapping type:

```go
type MySQLConnectionString string
```

## Advanced Features

### Binding Interfaces

Frequently, dependency injection is used to bind concrete implementations for an
interface. goose matches inputs to outputs via [type identity][], so the
inclination might be to create a provider that returns an interface type.
However, this would not be idiomatic, since the Go best practice is to [return
concrete types][]. Instead, you can declare an interface binding in a
provider set:

```go
type Fooer interface {
	Foo() string
}

type Bar string

func (b *Bar) Foo() string {
	return string(*b)
}

//goose:provide BarFooer
func provideBar() *Bar {
	b := new(Bar)
	*b = "Hello, World!"
	return b
}

//goose:bind BarFooer Fooer *Bar
```

The syntax is provider set name, interface type, and finally the concrete type.
An interface binding does not necessarily need to have a provider in the same
set that provides the concrete type.

[type identity]: https://golang.org/ref/spec#Type_identity
[return concrete types]: https://github.com/golang/go/wiki/CodeReviewComments#interfaces

### Optional Inputs

A provider input can be marked optional using `goose:optional`:

```go
//goose:provide Bar
//goose:optional foo

func provideBar(foo Foo) Bar {
	// ...
}
```

If used as part of an injector that does not bring in the `Foo` dependency, then
the injector will pass the provider the zero value as the `foo` argument.

### Struct Providers

Structs can also be marked as providers. Instead of calling a function, an
injector will fill in each field using the corresponding provider. For a given
struct type `S`, this would provide both `S` and `*S`. For example, given the
following providers:

```go
type Foo int
type Bar int

//goose:provide Foo

func provideFoo() Foo {
	// ...
}

//goose:provide Bar

func provideBar() Bar {
	// ...
}

//goose:provide

type FooBar struct {
	Foo Foo
	Bar Bar
}
```

A generated injector for `FooBar` would look like this:

```go
func injectFooBar() FooBar {
	foo := provideFoo()
	bar := provideBar()
	fooBar := FooBar{
		Foo: foo,
		Bar: bar,
	}
	return fooBar
}
```

And similarly if the injector needed a `*FooBar`.

Like function providers, you can mark dependencies of a struct provider optional
by using the `goose:optional` directive with the field names.

### Cleanup functions

If a provider creates a value that needs to be cleaned up (e.g. closing a file),
then it can return a closure to clean up the resource. The injector will use
this to either return an aggregated cleanup function to the caller or to clean
up the resource if a later provider returns an error.

```go
//goose:provide

func provideFile(log Logger, path Path) (*os.File, func(), error) {
	f, err := os.Open(string(path))
	if err != nil {
		return nil, nil, err
	}
	cleanup := func() {
		if err := f.Close(); err != nil {
			log.Log(err)
		}
	}
	return f, cleanup, nil
}
```

A cleanup function is guaranteed to be called before the cleanup function of any
of the provider's inputs and must have the signature `func()`.

## Future Work

-   Support for map bindings.
-   Support for multiple provider outputs.
-   Tighter validation for a provider set (cycles in unused providers goes
    unreported currently)
-   Visualization for provider sets