# Wire: Compile-Time Dependency Injection for Go

Wire is a static [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 Wire is the **provider**: a function that can
produce a value. These functions are ordinary Go code.

```go
package foobarbaz

type Foo struct {
	X int
}

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

Providers can specify dependencies with parameters:

```go
package foobarbaz

// ...

type Bar struct {
	X int
}

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

Providers can also return errors:

```go
package foobarbaz

import (
	"context"
	"errors"
)

// ...

type Baz struct {
	X int
}

// 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{X: bar.X}, nil
}
```

Providers can be grouped in **provider sets**.  To add these providers to a new
set called `SuperSet`, use the `wire.NewSet` function:

```go
package foobarbaz

import (
	// ...
	"github.com/google/go-cloud/wire"
)

// ...

var SuperSet = wire.NewSet(ProvideFoo, ProvideBar, ProvideBaz)
```

You can also add other provider sets into a provider set.

```go
package foobarbaz

import (
	// ...
	"example.com/some/other/pkg"
)

// ...

var MegaSet = wire.NewSet(SuperSet, pkg.OtherSet)
```

### Injectors

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

An injector is declared by writing a function declaration whose body is a call
to `wire.Build`. The return values don't matter as long as they are of the
correct type. The values themselves will be ignored in the generated code. 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 wireinject

// ^ build tag makes sure the stub is not built in the final build

package main

import (
	"context"

	"github.com/google/go-cloud/wire"
	"example.com/foobarbaz"
)

func initializeApp(ctx context.Context) (foobarbaz.Baz, error) {
	wire.Build(foobarbaz.MegaSet)
	return foobarbaz.Baz{}, nil
}
```

Like providers, injectors can be parameterized on inputs (which then get sent to
providers) and can return errors. Arguments to `wire.Build` are the same as
`wire.NewSet`: they form a provider set. This is the provider set that gets
used during code generation for that injector.

Any non-injector declarations found in a file with injectors will be copied into
the generated file.

You can generate the injector by invoking `gowire` in the package directory:

```
gowire
```

Or you can add the line `//go:generate gowire` 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`.)

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

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

//+build !wireinject

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 little dependency on Wire at runtime: all of the
written code is just normal Go code, and can be used without Wire.

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

## Best Practices

Wire is still not mature yet, but guidance that applies to Dagger generally
applies to Wire 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 Wire-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. Wire 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)
}

func ProvideBar() *Bar {
	b := new(Bar)
	*b = "Hello, World!"
	return b
}

var BarFooer = wire.NewSet(
	ProvideBar,
	wire.Bind(new(Fooer), new(Bar)))
```

The first argument to `wire.Bind` is a pointer to a value of the desired
interface type and the second argument is a zero value of the concrete type.
Any set that includes an interface binding must also 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

### 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

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

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

type FooBar struct {
	Foo Foo
	Bar Bar
}

var Set = wire.NewSet(
	ProvideFoo,
	ProvideBar,
	FooBar{})
```

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`.

### Binding Values

Occasionally, it is useful to bind a basic value (usually `nil`) to a type.
Instead of having injectors depend on a throwaway provider function, you can
add a value expression to a provider set.

```go
type Foo struct {
	X int
}

func injectFoo() Foo {
	wire.Build(wire.Value(Foo{X: 42}))
	return Foo{}
}
```

The generated injector would look like this:

```go
func injectFoo() Foo {
	foo := Foo{X: 42}
	return foo
}
```

It's important to note that the expression will be copied, so references to
variables will be evaluated during the call to the injector. `gowire` will emit
an error if the expression calls any functions.

### 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
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()`.

### Alternate Injector Syntax

If you grow weary of writing `return foobarbaz.Foo{}, nil` at the end of your
injector function declaration, you can instead write it more concisely with a
`panic`:

```go
func injectFoo() Foo {
	panic(wire.Build(/* ... */))
}
```