diff --git a/errors_test.go b/errors_test.go
index 38e1cdb..efd2ae7 100644
--- a/errors_test.go
+++ b/errors_test.go
@@ -28,6 +28,7 @@ func TestErrors(t *testing.T) {
 		{"MissingGroupStart.q", errors.MissingGroupStart},
 		{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "a"}},
 		{"UnknownIdentifier.q", &errors.UnknownIdentifier{Name: "x"}},
+		{"UnknownIdentifier2.q", &errors.UnknownIdentifier{Name: "x"}},
 	}
 
 	for _, test := range tests {
diff --git a/examples/hello/hello.q b/examples/hello/hello.q
index 60f65e4..d3e8c5c 100644
--- a/examples/hello/hello.q
+++ b/examples/hello/hello.q
@@ -3,8 +3,9 @@ main() {
 }
 
 hello() {
-	write := 1
-	stdout := 1
+	one := 1
+	write := one
+	stdout := one
 	address := 4194305
 	length := 3
 
diff --git a/src/build/Function.go b/src/build/Function.go
index 671c4e0..3ec442f 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -137,19 +137,35 @@ func (f *Function) CompileVariableDefinition(expr *expression.Expression) error
 
 	value := expr.Children[1]
 
+	err := value.EachLeaf(func(leaf *expression.Expression) error {
+		if leaf.Token.Kind == token.Identifier && !f.identifierExists(leaf.Token.Text()) {
+			return errors.New(&errors.UnknownIdentifier{Name: leaf.Token.Text()}, f.File, leaf.Token.Position)
+		}
+
+		return nil
+	})
+
+	if err != nil {
+		return err
+	}
+
 	// All expressions are returned to the memory pool.
 	// To avoid losing variable values, we will detach it from the expression.
 	expr.RemoveChild(value)
 
 	f.Variables[name] = &Variable{
-		Name:    name,
-		Value:   value,
-		IsConst: true,
+		Name:  name,
+		Value: value,
 	}
 
 	return nil
 }
 
+func (f *Function) identifierExists(name string) bool {
+	_, exists := f.Variables[name]
+	return exists
+}
+
 // CompileFunctionCall compiles a function call.
 func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
 	funcName := expr.Children[0].Token.Text()
@@ -193,18 +209,7 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 			return errors.New(&errors.UnknownIdentifier{Name: name}, f.File, t.Position)
 		}
 
-		if !variable.IsConst {
-			return errors.New(errors.NotImplemented, f.File, t.Position)
-		}
-
-		n, err := strconv.Atoi(variable.Value.Token.Text())
-
-		if err != nil {
-			return err
-		}
-
-		f.Assembler.MoveRegisterNumber(register, uint64(n))
-		return nil
+		return f.ExpressionToRegister(variable.Value, register)
 
 	case token.Number:
 		value := t.Text()
diff --git a/src/build/Variable.go b/src/build/Variable.go
index b5f7338..a37b2b5 100644
--- a/src/build/Variable.go
+++ b/src/build/Variable.go
@@ -4,7 +4,6 @@ import "git.akyoto.dev/cli/q/src/build/expression"
 
 // Variable represents a variable in a function.
 type Variable struct {
-	Name    string
-	Value   *expression.Expression
-	IsConst bool
+	Name  string
+	Value *expression.Expression
 }
diff --git a/src/build/expression/Expression.go b/src/build/expression/Expression.go
index 3cfbb97..37b7719 100644
--- a/src/build/expression/Expression.go
+++ b/src/build/expression/Expression.go
@@ -54,6 +54,23 @@ func (expr *Expression) Close() {
 	pool.Put(expr)
 }
 
+// EachLeaf iterates through all leaves in the tree.
+func (expr *Expression) EachLeaf(callBack func(*Expression) error) error {
+	if expr.IsLeaf() {
+		return callBack(expr)
+	}
+
+	for _, child := range expr.Children {
+		err := child.EachLeaf(callBack)
+
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 // RemoveChild removes a child from the expression.
 func (expr *Expression) RemoveChild(child *Expression) {
 	for i, c := range expr.Children {
diff --git a/src/errors/UnknownIdentifier.go b/src/errors/UnknownIdentifier.go
index d6f8dc8..fe1fc1f 100644
--- a/src/errors/UnknownIdentifier.go
+++ b/src/errors/UnknownIdentifier.go
@@ -11,8 +11,8 @@ type UnknownIdentifier struct {
 // Error generates the string representation.
 func (err *UnknownIdentifier) Error() string {
 	if err.CorrectName != "" {
-		return fmt.Sprintf("Unknown variable '%s', did you mean '%s'?", err.Name, err.CorrectName)
+		return fmt.Sprintf("Unknown identifier '%s', did you mean '%s'?", err.Name, err.CorrectName)
 	}
 
-	return fmt.Sprintf("Unknown variable '%s'", err.Name)
+	return fmt.Sprintf("Unknown identifier '%s'", err.Name)
 }
diff --git a/tests/errors/UnknownIdentifier2.q b/tests/errors/UnknownIdentifier2.q
new file mode 100644
index 0000000..4c60d29
--- /dev/null
+++ b/tests/errors/UnknownIdentifier2.q
@@ -0,0 +1,3 @@
+main() {
+	x := x
+}
\ No newline at end of file