Feature: Implement data type comparison

[alencarandre]

In order to compare between data types, I am doing

type (
  CustomInterface interface {
    Foo() string
  }

  CustomType1 struct { }
  CustomType2 struct { }
)

func (ct1 CustomType1) Foo() {}
func (ct2 CustomType2) Foo() {}

...

func TestType(t testing.T) {
   var i interface{} = &CustomType1{}
 
   _, isType1 := i.(CustomType1)
   _, isType2 := i.(CustomType1)

   assert.Assert(t, isType1) // Success
   assert.Assert(t, isType2) // Fail
}

It would be better if

customType := &CustomType1{}

assert.Assert(t, is.SameType(customType, CustomType1) // Success
assert.Assert(t, is.SameType(customType, CustomType2) // Fail

[dnephin]

A little while ago I considered adding something like Implements or Type. When I was experimenting with it I realized that most of the time if I cared about the type I probably wanted to use the type afterward.

Ex:

asCustomType, isType1 := i.(CustomType1)
assert.Assert(t, isType1)

asCustomType.Foo()

Pure type assertions didn't seem very useful because the compiler will already check the type for you. Do you have a more concrete example of where you would want to use this comparison but not use the type afterward?

[alencarandre]

Well, I implemented an interface and I am testing if the returned driver is correct type:

type (
  DataSource struct {
    Adapter string
  }

  DriverInterface interface {
    BlockList() []string
  }

  YamlDriver struct { }
  XmlDriver struct { }
)

func (yamlDriver *YamlDriver) BlockList() []string {
  ...
}

func (xmlDriver *XmlDriver) BlockList() []string {
  ...
}

func BuildDriver(DataSource ds) {
  ...
}

func TestBuildDriverYaml(t *testing.T) {
  var ds = NewDataSource()
  ds.Adapter = "yaml"

  var i interface{} = BuildDriver(ds)
  _, isYamlDriver := i.(*YamlDriver)

  assert.Assert(t, isYamlDriver)
}

[dnephin]

You can have the compiler perform that test for you by adding this line to a package:

var _ DriverInterface = &YamlDriver{}

I believe that's the recommended practice for checking a type implements an interface.

[dnephin]

A better example from #147

doer, ok := something.(Doer)
assert.Assert(t, ok, "got: %T", something)

var _ DriverInterface = (*YamlDriver)(nil)