diff --git a/src/build/Build.go b/src/build/Build.go
index fcf1698..1efb881 100644
--- a/src/build/Build.go
+++ b/src/build/Build.go
@@ -19,8 +19,8 @@ func New(files ...string) *Build {
 
 // Run parses the input files and generates an executable file.
 func (build *Build) Run() (map[string]*Function, error) {
-	functions, errors := Scan(build.Files)
-	return Compile(functions, errors)
+	functions, errors := scan(build.Files)
+	return compile(functions, errors)
 }
 
 // Executable returns the path to the executable.
diff --git a/src/build/Function.go b/src/build/Function.go
index 251d5e1..9b70b36 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -73,6 +73,123 @@ func (f *Function) Compile() {
 	f.Assembler.Return()
 }
 
+// CompileInstruction compiles a single instruction.
+func (f *Function) CompileInstruction(line token.List) error {
+	if len(line) == 0 {
+		return nil
+	}
+
+	if line[0].Kind == token.Keyword {
+		return f.CompileKeyword(line)
+	}
+
+	expr := expression.Parse(line)
+
+	if expr == nil {
+		return nil
+	}
+
+	defer expr.Close()
+
+	if isVariableDefinition(expr) {
+		return f.CompileVariableDefinition(expr)
+	}
+
+	if isFunctionCall(expr) {
+		return f.CompileFunctionCall(expr)
+	}
+
+	return errors.New(&errors.InvalidInstruction{Instruction: expr.Token.Text()}, f.File, expr.Token.Position)
+}
+
+// CompileKeyword compiles an instruction that starts with a keyword.
+func (f *Function) CompileKeyword(line token.List) error {
+	switch line[0].Text() {
+	case "return":
+		if len(line) > 1 {
+			value := expression.Parse(line[1:])
+			defer value.Close()
+			// TODO: Set the return value
+		}
+
+		f.Assembler.Return()
+
+	default:
+		return errors.New(&errors.KeywordNotImplemented{Keyword: line[0].Text()}, f.File, line[0].Position)
+	}
+
+	return nil
+}
+
+// CompileVariableDefinition compiles a variable definition.
+func (f *Function) CompileVariableDefinition(expr *expression.Expression) error {
+	if len(expr.Children) < 2 {
+		return errors.New(errors.MissingAssignValue, f.File, expr.LastChild().Token.After())
+	}
+
+	name := expr.Children[0].Token.Text()
+	_, exists := f.Variables[name]
+
+	if exists {
+		return errors.New(&errors.VariableAlreadyExists{Name: name}, f.File, expr.Children[0].Token.Position)
+	}
+
+	value := expr.Children[1]
+
+	// 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,
+	}
+
+	return nil
+}
+
+// CompileFunctionCall compiles a function call.
+func (f *Function) CompileFunctionCall(expr *expression.Expression) error {
+	funcName := expr.Children[0].Token.Text()
+	parameters := expr.Children[1:]
+
+	for i, parameter := range parameters {
+		switch parameter.Token.Kind {
+		case token.Identifier:
+			name := parameter.Token.Text()
+			variable, exists := f.Variables[name]
+
+			if !exists {
+				panic("Unknown identifier " + name)
+			}
+
+			if !variable.IsConst {
+				panic("Not implemented yet")
+			}
+
+			n, _ := strconv.Atoi(variable.Value.Token.Text())
+			f.Assembler.MoveRegisterNumber(x64.SyscallArgs[i], uint64(n))
+
+		case token.Number:
+			value := parameter.Token.Text()
+			n, _ := strconv.Atoi(value)
+			f.Assembler.MoveRegisterNumber(x64.SyscallArgs[i], uint64(n))
+
+		default:
+			panic("Unknown expression")
+		}
+	}
+
+	if funcName == "syscall" {
+		f.Assembler.Syscall()
+	} else {
+		f.Assembler.Call(funcName)
+	}
+
+	return nil
+}
+
 // PrintAsm shows the assembly instructions.
 func (f *Function) PrintAsm() {
 	ansi.Bold.Println(f.Name)
@@ -92,111 +209,17 @@ func (f *Function) PrintAsm() {
 	ansi.Dim.Println("╰────────────────────────────────────────────────────────────")
 }
 
-// CompileInstruction compiles a single instruction.
-func (f *Function) CompileInstruction(line token.List) error {
-	if len(line) == 0 {
-		return nil
-	}
-
-	if line[0].Kind == token.Keyword {
-		switch line[0].Text() {
-		case "return":
-			if len(line) > 1 {
-				value := expression.Parse(line[1:])
-				defer value.Close()
-				// TODO: Set the return value
-			}
-
-			f.Assembler.Return()
-
-		default:
-			return errors.New(&errors.KeywordNotImplemented{Keyword: line[0].Text()}, f.File, line[0].Position)
-		}
-	}
-
-	expr := expression.Parse(line)
-
-	if expr == nil {
-		return nil
-	}
-
-	defer expr.Close()
-
-	if expr.Token.Kind == token.Number || expr.Token.Kind == token.Identifier || expr.Token.Kind == token.String {
-		return errors.New(&errors.InvalidInstruction{Instruction: expr.Token.Text()}, f.File, expr.Token.Position)
-	}
-
-	if expr.Token.Text() == ":=" {
-		if len(expr.Children) < 2 {
-			return errors.New(errors.MissingAssignValue, f.File, expr.LastChild().Token.After())
-		}
-
-		name := expr.Children[0].Token.Text()
-		_, exists := f.Variables[name]
-
-		if exists {
-			return errors.New(&errors.VariableAlreadyExists{Name: name}, f.File, expr.Children[0].Token.Position)
-		}
-
-		value := expr.Children[1]
-
-		// All expressions are returned to the memory pool.
-		// To avoid losing variable values, we will remove it from the expression.
-		expr.RemoveChild(value)
-
-		f.Variables[name] = &Variable{
-			Name:    name,
-			Value:   value,
-			IsConst: true,
-		}
-
-		return nil
-	}
-
-	if expr.Token.Text() == "λ" {
-		funcName := expr.Children[0].Token.Text()
-		parameters := expr.Children[1:]
-
-		for i, parameter := range parameters {
-			switch parameter.Token.Kind {
-			case token.Identifier:
-				name := parameter.Token.Text()
-				variable, exists := f.Variables[name]
-
-				if !exists {
-					panic("Unknown identifier " + name)
-				}
-
-				if !variable.IsConst {
-					panic("Not implemented yet")
-				}
-
-				n, _ := strconv.Atoi(variable.Value.Token.Text())
-				f.Assembler.MoveRegisterNumber(x64.SyscallArgs[i], uint64(n))
-
-			case token.Number:
-				value := parameter.Token.Text()
-				n, _ := strconv.Atoi(value)
-				f.Assembler.MoveRegisterNumber(x64.SyscallArgs[i], uint64(n))
-
-			default:
-				panic("Unknown expression")
-			}
-		}
-
-		if funcName == "syscall" {
-			f.Assembler.Syscall()
-		} else {
-			f.Assembler.Call(funcName)
-		}
-
-		return nil
-	}
-
-	return nil
-}
-
 // String returns the function name.
 func (f *Function) String() string {
 	return f.Name
 }
+
+// isVariableDefinition returns true if the expression is a variable definition.
+func isVariableDefinition(expr *expression.Expression) bool {
+	return expr.Token.Kind == token.Operator && expr.Token.Text() == ":="
+}
+
+// isFunctionCall returns true if the expression is a function call.
+func isFunctionCall(expr *expression.Expression) bool {
+	return expr.Token.Kind == token.Operator && expr.Token.Text() == "λ"
+}
diff --git a/src/build/Compile.go b/src/build/compile.go
similarity index 84%
rename from src/build/Compile.go
rename to src/build/compile.go
index f6f1736..528fa43 100644
--- a/src/build/Compile.go
+++ b/src/build/compile.go
@@ -4,8 +4,8 @@ import (
 	"sync"
 )
 
-// Compile compiles all the functions.
-func Compile(functions <-chan *Function, errors <-chan error) (map[string]*Function, error) {
+// compile waits for all functions to finish compilation.
+func compile(functions <-chan *Function, errors <-chan error) (map[string]*Function, error) {
 	allFunctions := map[string]*Function{}
 
 	for functions != nil || errors != nil {
diff --git a/src/build/Scan.go b/src/build/scan.go
similarity index 97%
rename from src/build/Scan.go
rename to src/build/scan.go
index b3e808a..4d16bb0 100644
--- a/src/build/Scan.go
+++ b/src/build/scan.go
@@ -11,8 +11,8 @@ import (
 	"git.akyoto.dev/cli/q/src/errors"
 )
 
-// Scan scans the directory.
-func Scan(files []string) (<-chan *Function, <-chan error) {
+// scan scans the directory.
+func scan(files []string) (<-chan *Function, <-chan error) {
 	functions := make(chan *Function)
 	errors := make(chan error)