diff --git a/backend_wat.go b/backend_wat.go
index 8370115..307518c 100644
--- a/backend_wat.go
+++ b/backend_wat.go
@@ -303,7 +303,19 @@ func (c *Compiler) compileExpressionWAT(expr Expression) (string, error) {
 	switch expr.Type {
 	case Expression_Assignment:
 		ass := expr.Value.(AssignmentExpression)
-		return c.compileAssignmentExpressionWAT(ass)
+
+		if ass.Operation == Operation_Nop {
+			return c.compileAssignmentExpressionWAT(ass)
+		}
+
+		watRight, err := c.compileExpressionWAT(ass.Value)
+		if err != nil {
+			return "", err
+		}
+
+		updateOp := c.compileOperationWAT(ass.Operation, ass.Lhs.ValueType.Value.(PrimitiveType))
+
+		return c.compileAssignmentUpdateExpressionWAT(ass.Lhs, watRight+updateOp, false)
 	case Expression_Literal:
 		lit := expr.Value.(LiteralExpression)
 		switch lit.Literal.Type {
@@ -345,41 +357,7 @@ func (c *Compiler) compileExpressionWAT(expr Expression) (string, error) {
 			return "", err
 		}
 
-		op := ""
-
-		suffix := ""
-		if isUnsignedInt(operandType) {
-			suffix = "u"
-		} else {
-			suffix = "s"
-		}
-
-		switch binary.Operation {
-		case Operation_Add:
-			op = getPrimitiveWATType(operandType) + ".add\n"
-		case Operation_Sub:
-			op = getPrimitiveWATType(operandType) + ".sub\n"
-		case Operation_Mul:
-			op = getPrimitiveWATType(operandType) + ".mul\n"
-		case Operation_Div:
-			op = getPrimitiveWATType(operandType) + ".div_" + suffix + "\n"
-		case Operation_Mod:
-			op = getPrimitiveWATType(operandType) + ".rem_" + suffix + "\n"
-		case Operation_Greater:
-			op = getPrimitiveWATType(operandType) + ".gt_" + suffix + "\n"
-		case Operation_Less:
-			op = getPrimitiveWATType(operandType) + ".lt_" + suffix + "\n"
-		case Operation_GreaterEquals:
-			op = getPrimitiveWATType(operandType) + ".ge_" + suffix + "\n"
-		case Operation_LessEquals:
-			op = getPrimitiveWATType(operandType) + ".le_" + suffix + "\n"
-		case Operation_NotEquals:
-			op = getPrimitiveWATType(operandType) + ".ne\n"
-		case Operation_Equals:
-			op = getPrimitiveWATType(operandType) + ".eq\n"
-		default:
-			panic("operation not implemented")
-		}
+		op := c.compileOperationWAT(binary.Operation, operandType)
 
 		return watLeft + watRight + op + getTypeCast(exprType), nil
 	case Expression_Tuple:
@@ -482,6 +460,46 @@ func (c *Compiler) compileExpressionWAT(expr Expression) (string, error) {
 	panic("expr not implemented")
 }
 
+func (c *Compiler) compileOperationWAT(operation Operation, operandType PrimitiveType) string {
+	op := ""
+
+	suffix := ""
+	if isUnsignedInt(operandType) {
+		suffix = "u"
+	} else {
+		suffix = "s"
+	}
+
+	switch operation {
+	case Operation_Add:
+		op = getPrimitiveWATType(operandType) + ".add\n"
+	case Operation_Sub:
+		op = getPrimitiveWATType(operandType) + ".sub\n"
+	case Operation_Mul:
+		op = getPrimitiveWATType(operandType) + ".mul\n"
+	case Operation_Div:
+		op = getPrimitiveWATType(operandType) + ".div_" + suffix + "\n"
+	case Operation_Mod:
+		op = getPrimitiveWATType(operandType) + ".rem_" + suffix + "\n"
+	case Operation_Greater:
+		op = getPrimitiveWATType(operandType) + ".gt_" + suffix + "\n"
+	case Operation_Less:
+		op = getPrimitiveWATType(operandType) + ".lt_" + suffix + "\n"
+	case Operation_GreaterEquals:
+		op = getPrimitiveWATType(operandType) + ".ge_" + suffix + "\n"
+	case Operation_LessEquals:
+		op = getPrimitiveWATType(operandType) + ".le_" + suffix + "\n"
+	case Operation_NotEquals:
+		op = getPrimitiveWATType(operandType) + ".ne\n"
+	case Operation_Equals:
+		op = getPrimitiveWATType(operandType) + ".eq\n"
+	default:
+		panic("operation not implemented")
+	}
+
+	return op
+}
+
 func (c *Compiler) compileStatementWAT(stmt Statement, block *Block) (string, error) {
 	switch stmt.Type {
 	case Statement_Expression:
diff --git a/example/test.ely b/example/test.ely
index a1653bb..4cb5340 100644
--- a/example/test.ely
+++ b/example/test.ely
@@ -33,3 +33,8 @@ u64 fib(u64 i) {
 u64 sub(u64 a) {
 	return --raw(u64, 0xFFu64);
 }
+
+u64 assign(u64 a) {
+	a += 1u64;
+	return raw(u64, a) += 2u64;
+}
diff --git a/parser.go b/parser.go
index 9c1fef9..cad9b12 100644
--- a/parser.go
+++ b/parser.go
@@ -101,8 +101,9 @@ type Expression struct {
 }
 
 type AssignmentExpression struct {
-	Lhs   Expression
-	Value Expression
+	Lhs       Expression
+	Value     Expression
+	Operation Operation
 }
 
 type LiteralExpression struct {
@@ -127,6 +128,7 @@ const (
 	Operation_LessEquals
 	Operation_NotEquals
 	Operation_Equals
+	Operation_Nop
 )
 
 type UnaryOperation uint32
@@ -747,14 +749,10 @@ func (p *Parser) tryAssignmentExpression() (*Expression, error) {
 		return nil, err
 	}
 
-	if *op != Operator_Equals {
-		// TODO: incorrect, evaluates lhs twice
-		operation := getOperation(*op)
-		expr = &Expression{Type: Expression_Binary, Value: BinaryExpression{Left: *lhs, Right: *expr, Operation: operation}, Position: lhs.Position}
-	}
+	operation := getOperation(*op)
 
 	*p = pCopy
-	return &Expression{Type: Expression_Assignment, Value: AssignmentExpression{Lhs: *lhs, Value: *expr}, Position: lhs.Position}, nil
+	return &Expression{Type: Expression_Assignment, Value: AssignmentExpression{Lhs: *lhs, Value: *expr, Operation: operation}, Position: lhs.Position}, nil
 }
 
 func (p *Parser) tryExpression() (*Expression, error) {
diff --git a/types.go b/types.go
index 164d681..a38bf12 100644
--- a/types.go
+++ b/types.go
@@ -172,6 +172,8 @@ func getOperation(operator Operator) Operation {
 		return Operation_LessEquals
 	case Operator_NotEquals:
 		return Operation_NotEquals
+	case Operator_Equals:
+		return Operation_Nop
 	default:
 		return InvalidValue
 	}
diff --git a/validator.go b/validator.go
index 4e386cc..1c6ac4d 100644
--- a/validator.go
+++ b/validator.go
@@ -186,6 +186,11 @@ func (v *Validator) validatePotentiallyVoidExpression(expr *Expression) []error
 			return errors
 		}
 
+		if assignment.Operation != Operation_Equals && (assignment.Lhs.ValueType.Type != Type_Primitive || assignment.Value.ValueType.Type != Type_Primitive) {
+			errors = append(errors, v.createError("both sides of an arithmetic expression must evaluate to a primitive type", expr.Position))
+			return errors
+		}
+
 		if !isSameType(*assignment.Value.ValueType, *assignment.Lhs.ValueType) {
 			if !isTypeExpandableTo(*assignment.Value.ValueType, *assignment.Lhs.ValueType) {
 				errors = append(errors, v.createError(fmt.Sprintf("cannot assign expression of type %s to variable of type %s", *assignment.Value.ValueType, *assignment.Lhs.ValueType), expr.Position))