use errors.As instead of type assertion in IsError functions

This allows IsRedirect to detect a Redirect error even if
it's wrapped via fmt.Errorf("some description: %w", err).

error.go

@@ -21,7 +21,8 @@ func (m Method) Error() string {
 
 // IsMethod reports if err is considered a method error, returning it if so.
 func IsMethod(err error) (Method, bool) {
-	e, ok := err.(Method)
+	var e Method
+	ok := errors.As(err, &e)
 	return e, ok
 }
 
@@ -34,7 +35,8 @@ func (r Redirect) Error() string { return fmt.Sprintf("redirecting to %s", r.URL
 
 // IsRedirect reports if err is considered a redirect, returning it if so.
 func IsRedirect(err error) (Redirect, bool) {
-	e, ok := err.(Redirect)
+	var e Redirect
+	ok := errors.As(err, &e)
 	return e, ok
 }
 
@@ -48,7 +50,8 @@ func (b BadRequest) Error() string { return b.Err.Error() }
 
 // IsBadRequest reports if err is considered a bad request error, returning it if so.
 func IsBadRequest(err error) (BadRequest, bool) {
-	e, ok := err.(BadRequest)
+	var e BadRequest
+	ok := errors.As(err, &e)
 	return e, ok
 }
 
@@ -63,7 +66,8 @@ func (h HTTP) Error() string { return h.Err.Error() }
 
 // IsHTTP reports if err is considered an HTTP error, returning it if so.
 func IsHTTP(err error) (HTTP, bool) {
-	e, ok := err.(HTTP)
+	var e HTTP
+	ok := errors.As(err, &e)
 	return e, ok
 }
 
@@ -76,6 +80,7 @@ func (JSONResponse) Error() string { return "JSONResponse" }
 
 // IsJSONResponse reports if err is considered a JSON response, returning it if so.
 func IsJSONResponse(err error) (JSONResponse, bool) {
-	e, ok := err.(JSONResponse)
+	var e JSONResponse
+	ok := errors.As(err, &e)
 	return e, ok
 }

error_test.go

@@ -0,0 +1,28 @@
+package httperror_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/shurcooL/httperror"
+)
+
+func TestIsRedirect(t *testing.T) {
+	for _, tt := range [...]struct {
+		name string
+		in   error
+		want bool
+	}{
+		{name: "bare", in: httperror.Redirect{URL: "u"}, want: true},
+		{name: "wrapped in %w", in: fmt.Errorf("desc: %w", httperror.Redirect{URL: "u"}), want: true},
+		{name: "wrapped in %v", in: fmt.Errorf("desc: %v", httperror.Redirect{URL: "u"}), want: false},
+		{name: "not redirect", in: httperror.Method{Allowed: []string{"GET"}}, want: false},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			_, got := httperror.IsRedirect(tt.in)
+			if got != tt.want {
+				t.Errorf("got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

go.mod

@@ -0,0 +1,3 @@
+module github.com/shurcooL/httperror
+
+go 1.17