eval: Add a Cause function to safely extract the cause of exceptions.

eval/builtin_fn_flow.go

@@ -56,7 +56,7 @@ func each(fm *Frame, f Callable, inputs Inputs) error {
 		newFm.Close()
 
 		if ex != nil {
-			switch ex.(*Exception).Cause {
+			switch Cause(ex) {
 			case nil, Continue:
 				// nop
 			case Break:
@@ -87,7 +87,7 @@ func peach(fm *Frame, f Callable, inputs Inputs) error {
 			newFm.Close()
 
 			if ex != nil {
-				switch ex.(*Exception).Cause {
+				switch Cause(ex) {
 				case nil, Continue:
 					// nop
 				case Break:

eval/builtin_fn_str.go

@@ -159,7 +159,7 @@ func eawk(fm *Frame, f Callable, inputs Inputs) error {
 		newFm.Close()
 
 		if ex != nil {
-			switch ex.(*Exception).Cause {
+			switch Cause(ex) {
 			case nil, Continue:
 				// nop
 			case Break:

eval/builtin_special.go

@@ -212,7 +212,7 @@ type fnWrap struct{ wrapped effectOp }
 
 func (op fnWrap) invoke(fm *Frame) error {
 	err := fm.Eval(op.wrapped)
-	if err != nil && err.(*Exception).Cause != Return {
+	if err != nil && Cause(err) != Return {
 		// rethrow
 		return err
 	}

eval/exception.go

@@ -22,6 +22,15 @@ type Exception struct {
 	Traceback *stackTrace
 }
 
+// Cause returns the Cause field if err is an *Exception. Otherwise it returns
+// err itself.
+func Cause(err error) error {
+	if exc, ok := err.(*Exception); ok {
+		return exc.Cause
+	}
+	return err
+}
+
 // OK is a pointer to the zero value of Exception, representing the absence of
 // exception.
 var OK = &Exception{}

eval/exception_test.go

@@ -1,6 +1,20 @@
 package eval
 
-import "testing"
+import (
+	"errors"
+	"testing"
+
+	"github.com/elves/elvish/tt"
+)
+
+func TestCause(t *testing.T) {
+	err := errors.New("ordinary error")
+	exc := &Exception{Cause: err}
+	tt.Test(t, tt.Fn("Cause", Cause), tt.Table{
+		tt.Args(err).Rets(err),
+		tt.Args(exc).Rets(err),
+	})
+}
 
 func TestException(t *testing.T) {
 	Test(t,

eval/testutils.go

@@ -223,7 +223,7 @@ func matchErr(want, got error) bool {
 	if got == nil {
 		return want == nil
 	}
-	return want == errAny || reflect.DeepEqual(got.(*Exception).Cause, want)
+	return want == errAny || reflect.DeepEqual(Cause(got), want)
 }
 
 // MustMkdirAll calls os.MkdirAll and panics if an error is returned. It is