Merge pull request #60 from tlm/as-type-support

Adds support for a new convenience function AsType and HasType.

functions.go

@@ -351,18 +351,17 @@ func Is(err, target error) bool {
 	return stderrors.Is(err, target)
 }
 
-// IsType is a convenience method for ascertaining if an error contains the
-// target error type within its chain. This is aimed at ease of development
-// where a more complicated error type wants to be to checked for existence but
-// pointer var of that type is too much overhead.
-func IsType[t error](err error) bool {
-	for err != nil {
-		if _, is := err.(t); is {
-			return true
-		}
-		err = stderrors.Unwrap(err)
-	}
-	return false
+// HasType is a function wrapper around AsType dropping the where return value
+// from AsType() making a function that can be used like this:
+//
+//  return HasType[*MyError](err)
+//
+// Or
+//
+//  if HasType[*MyError](err) {}
+func HasType[T error](err error) bool {
+	_, rval := AsType[T](err)
+	return rval
 }
 
 // As is a proxy for the As function in Go's standard `errors` library
@@ -371,6 +370,44 @@ func As(err error, target interface{}) bool {
 	return stderrors.As(err, target)
 }
 
+// AsType is a convenience method for checking and getting an error from within
+// a chain that is of type T. If no error is found of type T in the chain the
+// zero value of T is returned with false. If an error in the chain implementes
+// As(any) bool then it's As method will be called if it's type is not of type T.
+
+// AsType finds the first error in err's chain that is assignable to type T, and
+// if a match is found, returns that error value and true. Otherwise, it returns
+// T's zero value and false.
+//
+// AsType is equivalent to errors.As, but uses a type parameter and returns
+// the target, to avoid having to define a variable before the call. For
+// example, callers can replace this:
+//
+//  var pathError *fs.PathError
+//  if errors.As(err, &pathError) {
+//      fmt.Println("Failed at path:", pathError.Path)
+//  }
+//
+// With:
+//
+//  if pathError, ok := errors.AsType[*fs.PathError](err); ok {
+//      fmt.Println("Failed at path:", pathError.Path)
+//  }
+func AsType[T error](err error) (T, bool) {
+	for err != nil {
+		if e, is := err.(T); is {
+			return e, true
+		}
+		var res T
+		if x, ok := err.(interface{ As(any) bool }); ok && x.As(&res) {
+			return res, true
+		}
+		err = stderrors.Unwrap(err)
+	}
+	var zero T
+	return zero, false
+}
+
 // SetLocation takes a given error and records where in the stack SetLocation
 // was called from and returns the wrapped error with the location information
 // set. The returned error implements the Locationer interface. If err is nil

functions_test.go

@@ -403,11 +403,9 @@ func (*functionSuite) TestQuietWrappedErrorStillSatisfied(c *gc.C) {
 	c.Assert(errors.Is(err, simpleTestError), gc.Equals, true)
 }
 
-type FooError struct {
-}
-
-func (*FooError) Error() string {
-	return "I am here boss"
+type ComplexErrorMessage interface {
+	error
+	ComplexMessage() string
 }
 
 type complexError struct {
@@ -418,30 +416,90 @@ func (c *complexError) Error() string {
 	return c.Message
 }
 
+func (c *complexError) ComplexMessage() string {
+	return c.Message
+}
+
 type complexErrorOther struct {
 	Message string
 }
 
+func (c *complexErrorOther) As(e any) bool {
+	if ce, ok := e.(**complexError); ok {
+		*ce = &complexError{
+			Message: c.Message,
+		}
+		return true
+	}
+	return false
+}
+
 func (c *complexErrorOther) Error() string {
 	return c.Message
 }
 
-func (*functionSuite) TestIsType(c *gc.C) {
+func (c *complexErrorOther) ComplexMessage() string {
+	return c.Message
+}
+
+func (*functionSuite) TestHasType(c *gc.C) {
 	complexErr := &complexError{Message: "complex error message"}
 	wrapped1 := fmt.Errorf("wrapping1: %w", complexErr)
 	wrapped2 := fmt.Errorf("wrapping2: %w", wrapped1)
 
-	c.Assert(errors.IsType[*complexError](complexErr), gc.Equals, true)
-	c.Assert(errors.IsType[*complexError](wrapped1), gc.Equals, true)
-	c.Assert(errors.IsType[*complexError](wrapped2), gc.Equals, true)
-	c.Assert(errors.IsType[*complexErrorOther](complexErr), gc.Equals, false)
-	c.Assert(errors.IsType[*complexErrorOther](wrapped1), gc.Equals, false)
-	c.Assert(errors.IsType[*complexErrorOther](wrapped2), gc.Equals, false)
+	c.Assert(errors.HasType[*complexError](complexErr), gc.Equals, true)
+	c.Assert(errors.HasType[*complexError](wrapped1), gc.Equals, true)
+	c.Assert(errors.HasType[*complexError](wrapped2), gc.Equals, true)
+	c.Assert(errors.HasType[ComplexErrorMessage](wrapped2), gc.Equals, true)
+	c.Assert(errors.HasType[*complexErrorOther](wrapped2), gc.Equals, false)
+	c.Assert(errors.HasType[*complexErrorOther](nil), gc.Equals, false)
 
-	err := errors.New("test")
-	c.Assert(errors.IsType[*complexErrorOther](err), gc.Equals, false)
+	complexErrOther := &complexErrorOther{Message: "another complex error"}
+
+	c.Assert(errors.HasType[*complexError](complexErrOther), gc.Equals, true)
 
-	c.Assert(errors.IsType[*complexErrorOther](nil), gc.Equals, false)
+	wrapped2 = fmt.Errorf("wrapping1: %w", complexErrOther)
+	c.Assert(errors.HasType[*complexError](wrapped2), gc.Equals, true)
+}
+
+func (*functionSuite) TestAsType(c *gc.C) {
+	complexErr := &complexError{Message: "complex error message"}
+	wrapped1 := fmt.Errorf("wrapping1: %w", complexErr)
+	wrapped2 := fmt.Errorf("wrapping2: %w", wrapped1)
+
+	ce, ok := errors.AsType[*complexError](complexErr)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(ce.Message, gc.Equals, complexErr.Message)
+
+	ce, ok = errors.AsType[*complexError](wrapped1)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(ce.Message, gc.Equals, complexErr.Message)
+
+	ce, ok = errors.AsType[*complexError](wrapped2)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(ce.Message, gc.Equals, complexErr.Message)
+
+	cem, ok := errors.AsType[ComplexErrorMessage](wrapped2)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(cem.ComplexMessage(), gc.Equals, complexErr.Message)
+
+	ceo, ok := errors.AsType[*complexErrorOther](wrapped2)
+	c.Assert(ok, gc.Equals, false)
+	c.Assert(ceo, gc.Equals, (*complexErrorOther)(nil))
+
+	ceo, ok = errors.AsType[*complexErrorOther](nil)
+	c.Assert(ok, gc.Equals, false)
+	c.Assert(ceo, gc.Equals, (*complexErrorOther)(nil))
+
+	complexErrOther := &complexErrorOther{Message: "another complex error"}
+	ce, ok = errors.AsType[*complexError](complexErrOther)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(ce.Message, gc.Equals, complexErrOther.Message)
+
+	wrapped2 = fmt.Errorf("wrapping1: %w", complexErrOther)
+	ce, ok = errors.AsType[*complexError](wrapped2)
+	c.Assert(ok, gc.Equals, true)
+	c.Assert(ce.Message, gc.Equals, complexErrOther.Message)
 }
 
 func ExampleHide() {
@@ -464,12 +522,24 @@ func (m *MyError) Error() string {
 	return m.Message
 }
 
-func ExampleIsType() {
+func ExampleHasType() {
+	myErr := &MyError{Message: "these are not the droids you're looking for"}
+	err := fmt.Errorf("wrapped: %w", myErr)
+	is := errors.HasType[*MyError](err)
+	fmt.Println(is)
+
+	// Output:
+	// true
+}
+
+func ExampleAsType() {
 	myErr := &MyError{Message: "these are not the droids you're looking for"}
 	err := fmt.Errorf("wrapped: %w", myErr)
-	is := errors.IsType[*MyError](err)
+	myErr, is := errors.AsType[*MyError](err)
 	fmt.Println(is)
+	fmt.Println(myErr.Message)
 
 	// Output:
 	// true
+	// these are not the droids you're looking for
 }