diff --git a/pkg/lambda/expression.go b/pkg/lambda/expression.go
index affaf54..f34b3b4 100644
--- a/pkg/lambda/expression.go
+++ b/pkg/lambda/expression.go
@@ -30,12 +30,12 @@ func NewAbstraction(parameter string, body Expression) *Abstraction {
 /** ------------------------------------------------------------------------- */
 
 type Application struct {
-	function Expression
-	argument Expression
+	abstraction Expression
+	argument    Expression
 }
 
-func (a *Application) Function() Expression {
-	return a.function
+func (a *Application) Abstraction() Expression {
+	return a.abstraction
 }
 
 func (a *Application) Argument() Expression {
@@ -46,8 +46,8 @@ func (a *Application) Accept(v Visitor) {
 	v.VisitApplication(a)
 }
 
-func NewApplication(function Expression, argument Expression) *Application {
-	return &Application{function: function, argument: argument}
+func NewApplication(abstraction Expression, argument Expression) *Application {
+	return &Application{abstraction: abstraction, argument: argument}
 }
 
 /** ------------------------------------------------------------------------- */
diff --git a/pkg/lambda/get_free_variables.go b/pkg/lambda/get_free_variables.go
index 72f39ac..1615aac 100644
--- a/pkg/lambda/get_free_variables.go
+++ b/pkg/lambda/get_free_variables.go
@@ -11,7 +11,7 @@ func GetFreeVariables(e Expression) *set.Set[string] {
 		vars.Remove(e.parameter)
 		return vars
 	case *Application:
-		vars := GetFreeVariables(e.function)
+		vars := GetFreeVariables(e.abstraction)
 		vars.Merge(GetFreeVariables(e.argument))
 		return vars
 	default:
diff --git a/pkg/lambda/is_free_variable.go b/pkg/lambda/is_free_variable.go
index 2aff5d4..abc9b4b 100644
--- a/pkg/lambda/is_free_variable.go
+++ b/pkg/lambda/is_free_variable.go
@@ -7,7 +7,7 @@ func IsFreeVariable(n string, e Expression) bool {
 	case *Abstraction:
 		return e.parameter != n && IsFreeVariable(n, e.body)
 	case *Application:
-		return IsFreeVariable(n, e.function) || IsFreeVariable(n, e.argument)
+		return IsFreeVariable(n, e.abstraction) || IsFreeVariable(n, e.argument)
 	default:
 		return false
 	}
diff --git a/pkg/lambda/reduce.go b/pkg/lambda/reduce.go
index 74deba2..55d366a 100644
--- a/pkg/lambda/reduce.go
+++ b/pkg/lambda/reduce.go
@@ -12,14 +12,14 @@ func ReduceOnce(e *Expression) bool {
 		case *Abstraction:
 			stack.Push(&typed.body)
 		case *Application:
-			if fn, fnOk := typed.function.(*Abstraction); fnOk {
+			if fn, fnOk := typed.abstraction.(*Abstraction); fnOk {
 				reduced := Substitute(fn.body, fn.parameter, typed.argument)
 				*top = reduced
 				return true
 			}
 
 			stack.Push(&typed.argument)
-			stack.Push(&typed.function)
+			stack.Push(&typed.abstraction)
 		}
 	}
 
diff --git a/pkg/lambda/rename.go b/pkg/lambda/rename.go
index 7a52796..bc5625c 100644
--- a/pkg/lambda/rename.go
+++ b/pkg/lambda/rename.go
@@ -23,14 +23,14 @@ func Rename(expr Expression, target string, newName string) Expression {
 		return NewAbstraction(newParam, newBody)
 
 	case *Application:
-		newFunc := Rename(e.function, target, newName)
+		newAbs := Rename(e.abstraction, target, newName)
 		newArg := Rename(e.argument, target, newName)
 
-		if newFunc == e.function && newArg == e.argument {
+		if newAbs == e.abstraction && newArg == e.argument {
 			return e
 		}
 
-		return NewApplication(newFunc, newArg)
+		return NewApplication(newAbs, newArg)
 
 	default:
 		return expr
diff --git a/pkg/lambda/stringify.go b/pkg/lambda/stringify.go
index 0769ed6..1d838b2 100644
--- a/pkg/lambda/stringify.go
+++ b/pkg/lambda/stringify.go
@@ -19,7 +19,7 @@ func (v *stringifyVisitor) VisitAbstraction(f *Abstraction) {
 
 func (v *stringifyVisitor) VisitApplication(c *Application) {
 	v.builder.WriteRune('(')
-	c.function.Accept(v)
+	c.abstraction.Accept(v)
 	v.builder.WriteRune(' ')
 	c.argument.Accept(v)
 	v.builder.WriteRune(')')
diff --git a/pkg/lambda/substitute.go b/pkg/lambda/substitute.go
index b89f8e0..fcfb043 100644
--- a/pkg/lambda/substitute.go
+++ b/pkg/lambda/substitute.go
@@ -31,14 +31,14 @@ func Substitute(expr Expression, target string, replacement Expression) Expressi
 		return NewAbstraction(param, newBody)
 
 	case *Application:
-		newFunc := Substitute(e.function, target, replacement)
+		newAbs := Substitute(e.abstraction, target, replacement)
 		newArg := Substitute(e.argument, target, replacement)
 
-		if newFunc == e.function && newArg == e.argument {
+		if newAbs == e.abstraction && newArg == e.argument {
 			return e
 		}
 
-		return NewApplication(newFunc, newArg)
+		return NewApplication(newAbs, newArg)
 
 	default:
 		return expr