Wrap panic errors

engine/builtin_test.go

@@ -33,6 +33,9 @@ func TestCall(t *testing.T) {
 			panic("told you")
 		})
 	})
+	vm.Register0(NewAtom("do_not_call_wrapped"), func(*VM, Cont, *Env) *Promise {
+		panic(errors.New("told you"))
+	})
 	assert.NoError(t, vm.Compile(context.Background(), `
 foo.
 foo(_, _).
@@ -60,8 +63,9 @@ f(g([a, [b, c|X]])).
 
 		{title: `cover all`, goal: atomComma.Apply(atomCut, NewAtom("f").Apply(NewAtom("g").Apply(List(NewAtom("a"), PartialList(NewVariable(), NewAtom("b"), NewAtom("c")))))), ok: true},
 		{title: `out of memory`, goal: NewAtom("foo").Apply(NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable(), NewVariable()), err: resourceError(resourceMemory, nil), mem: 1},
-		{title: `panic`, goal: NewAtom("do_not_call"), err: errors.New("panic: told you")},
-		{title: `panic (lazy)`, goal: NewAtom("lazy_do_not_call"), err: errors.New("panic: told you")},
+		{title: `panic`, goal: NewAtom("do_not_call"), err: PanicError{errors.New("told you")}},
+		{title: `panic (lazy)`, goal: NewAtom("lazy_do_not_call"), err: PanicError{errors.New("told you")}},
+		{title: `panic (wrapped)`, goal: NewAtom("do_not_call_wrapped"), err: PanicError{errors.New("told you")}},
 	}
 
 	for _, tt := range tests {

engine/promise.go

@@ -129,7 +129,12 @@
 }
 
 func panicError(r interface{}) error {
-	return fmt.Errorf("panic: %v", r)
+	switch r := r.(type) {
+	case error:
+		return PanicError{r}
+	default:
+		return PanicError{fmt.Errorf("%v", r)}
+	}
 }
 
 type promiseStack []*Promise
@@ -164,3 +169,12 @@
 	// went through all the ancestor promises and still got the unhandled error.
 	return err
 }
+
+// PanicError is an error thrown once panic occurs during the execution of a promise.
+type PanicError struct {
+	OriginErr error
+}
+
+func (p PanicError) Error() string {
+	return fmt.Sprintf("panic: %v", p.OriginErr)
+}

engine/variable.go

@@ -7,7 +7,7 @@ import (
 	"sync"
 )
 
-var errMaxVariables = errors.New("maximum number of variables reached")
+var ErrMaxVariables = errors.New("maximum number of variables reached")
 
 var maxVariables uint64
 var varCounter = struct {
@@ -31,7 +31,7 @@ func NewVariable() Variable {
 	defer varCounter.Unlock()
 	varCounter.Lock()
 	if maxVariables != 0 && varCounter.count >= maxVariables {
-		panic(errMaxVariables)
+		panic(ErrMaxVariables)
 	}
 	varCounter.count++
 	return Variable(varCounter.count)