Rename our UnmarshalTerraform5Type interface.

The names didn't make a whole lot of sense. "Unmarshal" evokes JSON or
other encoding schemes, which isn't really what we're doing here. Also,
we're not working with Types, we're working wtih Values.

By using FromTerraform5Value, we can mirror it with ToTerraform5Value,
and the name's are clearer about what the results will be and their
usage.

Author: Paddy Carver

tfprotov5/tftypes/value.go

@@ -8,8 +8,12 @@ import (
 	"github.com/vmihailenco/msgpack"
 )
 
-type Unmarshaler interface {
-	UnmarshalTerraform5Type(Value) error
+type ValueConverter interface {
+	FromTerraform5Value(Value) error
+}
+
+type ValueCreator interface {
+	ToTerraform5Value() (Value, error)
 }
 
 type ErrUnhandledType string
@@ -26,8 +30,6 @@ func (u msgPackUnknownType) MarshalMsgpack() ([]byte, error) {
 	return []byte{0xd4, 0, 0}, nil
 }
 
-// Value represents a form of a Terraform type that can be parsed into a Go
-// type.
 type Value struct {
 	typ   Type
 	value interface{}
@@ -41,9 +43,9 @@ func NewValue(t Type, val interface{}) Value {
 }
 
 func (val Value) As(dst interface{}) error {
-	unmarshaler, ok := dst.(Unmarshaler)
+	unmarshaler, ok := dst.(ValueConverter)
 	if ok {
-		return unmarshaler.UnmarshalTerraform5Type(val)
+		return unmarshaler.FromTerraform5Value(val)
 	}
 	if !val.IsKnown() {
 		return fmt.Errorf("unmarshaling unknown values is not supported")
@@ -135,7 +137,7 @@ func (val Value) As(dst interface{}) error {
 		}
 		return val.As(*target)
 	}
-	return fmt.Errorf("can't unmarshal into %T, needs UnmarshalTerraform5Type method", dst)
+	return fmt.Errorf("can't unmarshal into %T, needs FromTerraform5Value method", dst)
 }
 
 func (val Value) Is(t Type) bool {

tfprotov5/tftypes/value_example_test.go

@@ -63,9 +63,9 @@ type exampleResource struct {
 	suppliedName *bool // true for yes, false for no, nil for we haven't asked
 }
 
-// fill the tftypes.Unmarshaler interface to control how As works
+// fill the tftypes.ValueConverter interface to control how As works
 // we want a pointer to exampleResource so we can change the properties
-func (e *exampleResource) UnmarshalTerraform5Type(val tftypes.Value) error {
+func (e *exampleResource) FromTerraform5Value(val tftypes.Value) error {
 	// this is an object type, so we're always going to get a
 	// `tftypes.Value` that coerces to a map[string]tftypes.Value
 	// as input
@@ -105,10 +105,10 @@ func ExampleValue_As_interface() {
 		"supplied_name": tftypes.NewValue(tftypes.Bool, nil),
 	})
 
-	// exampleResource has UnmarshalTerraform5Type method defined on it,
-	// see value_example_test.go for implementation details
-	// we'd put the function and type inline here, but apparently Go can't
-	// have methods defined on types defined inside a function
+	// exampleResource has FromTerraform5Value method defined on it, see
+	// value_example_test.go for implementation details. We'd put the
+	// function and type inline here, but apparently Go can't have methods
+	// defined on types defined inside a function
 	var res exampleResource
 
 	// call As as usual