Fix operator overloading (#554)

conf/operators.go

@@ -48,12 +48,13 @@ func (p *OperatorPatcher) Visit(node *ast.Node) {
 	leftType := binaryNode.Left.Type()
 	rightType := binaryNode.Right.Type()
 
-	_, fn, ok := FindSuitableOperatorOverload(fns, p.Types, leftType, rightType)
+	ret, fn, ok := FindSuitableOperatorOverload(fns, p.Types, leftType, rightType)
 	if ok {
 		newNode := &ast.CallNode{
 			Callee:    &ast.IdentifierNode{Value: fn},
 			Arguments: []ast.Node{binaryNode.Left, binaryNode.Right},
 		}
+		newNode.SetType(ret)
 		ast.Patch(node, newNode)
 	}
 }

expr_test.go

@@ -311,6 +311,47 @@ func ExampleOperator() {
 	// Output: true
 }
 
+func ExampleOperator_Decimal() {
+	type Decimal struct{ N float64 }
+	code := `A + B - C`
+
+	type Env struct {
+		A, B, C   Decimal
+		Sub       func(a, b Decimal) Decimal
+		Add       func(a, b Decimal) Decimal
+	}
+
+	options := []expr.Option{
+		expr.Env(Env{}),
+		expr.Operator("+", "Add"),
+		expr.Operator("-", "Sub"),
+	}
+
+	program, err := expr.Compile(code, options...)
+	if err != nil {
+		fmt.Printf("Compile error: %v", err)
+		return
+	}
+
+	env := Env{
+		A: Decimal{3},
+		B: Decimal{2},
+		C: Decimal{1},
+		Sub: func(a, b Decimal) Decimal { return Decimal{a.N - b.N} },
+		Add: func(a, b Decimal) Decimal { return Decimal{a.N + b.N}  },
+	}
+
+	output, err := expr.Run(program, env)
+	if err != nil {
+		fmt.Printf("%v", err)
+		return
+	}
+
+	fmt.Printf("%v", output)
+
+	// Output: {4}
+}
+
 func fib(n int) int {
 	if n <= 1 {
 		return n