diff --git a/src/build/core/Compare.go b/src/build/core/Compare.go
index 0ccfe3c..c884a82 100644
--- a/src/build/core/Compare.go
+++ b/src/build/core/Compare.go
@@ -25,7 +25,7 @@ func (f *Function) Compare(comparison *expression.Expression) error {
 	}
 
 	if ast.IsFunctionCall(left) && right.IsLeaf() {
-		err := f.CompileCall(left)
+		_, err := f.CompileCall(left)
 
 		if err != nil {
 			return err
@@ -35,7 +35,7 @@ func (f *Function) Compare(comparison *expression.Expression) error {
 	}
 
 	tmp := f.NewRegister()
-	err := f.ExpressionToRegister(left, tmp)
+	_, err := f.ExpressionToRegister(left, tmp)
 
 	if err != nil {
 		return err
diff --git a/src/build/core/CompileASTNode.go b/src/build/core/CompileASTNode.go
index 7277ccc..ddbd05a 100644
--- a/src/build/core/CompileASTNode.go
+++ b/src/build/core/CompileASTNode.go
@@ -17,7 +17,8 @@ func (f *Function) CompileASTNode(node ast.Node) error {
 
 	case *ast.Call:
 		f.Fold(node.Expression)
-		return f.CompileCall(node.Expression)
+		_, err := f.CompileCall(node.Expression)
+		return err
 
 	case *ast.Define:
 		f.Fold(node.Expression)
diff --git a/src/build/core/CompileAssignArray.go b/src/build/core/CompileAssignArray.go
index 167d3e3..8f812d5 100644
--- a/src/build/core/CompileAssignArray.go
+++ b/src/build/core/CompileAssignArray.go
@@ -33,5 +33,6 @@ func (f *Function) CompileAssignArray(node *ast.Assign) error {
 		Length: byte(1),
 	}
 
-	return f.ExpressionToMemory(right, memory)
+	_, err = f.ExpressionToMemory(right, memory)
+	return err
 }
diff --git a/src/build/core/CompileAssignDivision.go b/src/build/core/CompileAssignDivision.go
index 177b008..9a06c61 100644
--- a/src/build/core/CompileAssignDivision.go
+++ b/src/build/core/CompileAssignDivision.go
@@ -29,7 +29,7 @@ func (f *Function) CompileAssignDivision(node *ast.Assign) error {
 	}
 
 	dividend := right.Children[0]
-	dividendRegister, err := f.Evaluate(dividend)
+	_, dividendRegister, err := f.Evaluate(dividend)
 
 	if err != nil {
 		return err
diff --git a/src/build/core/CompileCall.go b/src/build/core/CompileCall.go
index c04f842..25466db 100644
--- a/src/build/core/CompileCall.go
+++ b/src/build/core/CompileCall.go
@@ -12,12 +12,14 @@ import (
 // All call registers must hold the correct parameter values before the function invocation.
 // Registers that are in use must be saved if they are modified by the function.
 // After the function call, they must be restored in reverse order.
-func (f *Function) CompileCall(root *expression.Expression) error {
+func (f *Function) CompileCall(root *expression.Expression) (*Function, error) {
 	var (
 		pkg      = f.Package
 		nameRoot = root.Children[0]
+		fn       *Function
 		name     string
 		fullName string
+		exists   bool
 	)
 
 	if nameRoot.IsLeaf() {
@@ -32,13 +34,13 @@ func (f *Function) CompileCall(root *expression.Expression) error {
 	if !isSyscall {
 		if pkg != f.File.Package {
 			if f.File.Imports == nil {
-				return errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
+				return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
 			}
 
 			imp, exists := f.File.Imports[pkg]
 
 			if !exists {
-				return errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
+				return nil, errors.New(&errors.UnknownPackage{Name: pkg}, f.File, nameRoot.Token.Position)
 			}
 
 			imp.Used = true
@@ -49,10 +51,10 @@ func (f *Function) CompileCall(root *expression.Expression) error {
 		tmp.WriteString(".")
 		tmp.WriteString(name)
 		fullName = tmp.String()
-		_, exists := f.Functions[fullName]
+		fn, exists = f.Functions[fullName]
 
 		if !exists {
-			return errors.New(&errors.UnknownFunction{Name: name}, f.File, nameRoot.Token.Position)
+			return nil, errors.New(&errors.UnknownFunction{Name: name}, f.File, nameRoot.Token.Position)
 		}
 	}
 
@@ -66,7 +68,7 @@ func (f *Function) CompileCall(root *expression.Expression) error {
 	err := f.ExpressionsToRegisters(parameters, registers)
 
 	if err != nil {
-		return err
+		return fn, err
 	}
 
 	// TODO: Save all return value registers of the function
@@ -104,5 +106,5 @@ func (f *Function) CompileCall(root *expression.Expression) error {
 		}
 	}
 
-	return nil
+	return fn, nil
 }
diff --git a/src/build/core/CompileDefinition.go b/src/build/core/CompileDefinition.go
index 689f020..aeed43d 100644
--- a/src/build/core/CompileDefinition.go
+++ b/src/build/core/CompileDefinition.go
@@ -5,6 +5,7 @@ import (
 	"git.akyoto.dev/cli/q/src/build/ast"
 	"git.akyoto.dev/cli/q/src/build/errors"
 	"git.akyoto.dev/cli/q/src/build/expression"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // CompileDefinition compiles a variable definition.
@@ -19,9 +20,20 @@ func (f *Function) CompileDefinition(node *ast.Define) error {
 			return err
 		}
 
-		err = f.ExpressionToRegister(right, variable.Register)
+		typ, err := f.ExpressionToRegister(right, variable.Register)
+
+		if err != nil {
+			return err
+		}
+
+		variable.Type = typ
+
+		if variable.Type == types.Invalid {
+			return errors.New(errors.UnknownType, f.File, node.Expression.Token.End())
+		}
+
 		f.AddVariable(variable)
-		return err
+		return nil
 	}
 
 	if !ast.IsFunctionCall(right) {
@@ -29,7 +41,7 @@ func (f *Function) CompileDefinition(node *ast.Define) error {
 	}
 
 	count := 0
-	err := f.CompileCall(right)
+	_, err := f.CompileCall(right)
 
 	if err != nil {
 		return err
diff --git a/src/build/core/Evaluate.go b/src/build/core/Evaluate.go
index f7d601a..145f3b1 100644
--- a/src/build/core/Evaluate.go
+++ b/src/build/core/Evaluate.go
@@ -4,21 +4,22 @@ import (
 	"git.akyoto.dev/cli/q/src/build/cpu"
 	"git.akyoto.dev/cli/q/src/build/expression"
 	"git.akyoto.dev/cli/q/src/build/token"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // Evaluate evaluates an expression and returns a register that contains the value of the expression.
-func (f *Function) Evaluate(expr *expression.Expression) (cpu.Register, error) {
+func (f *Function) Evaluate(expr *expression.Expression) (types.Type, cpu.Register, error) {
 	if expr.Token.Kind == token.Identifier {
 		name := expr.Token.Text(f.File.Bytes)
 		variable := f.VariableByName(name)
 
 		if variable.Alive == 1 {
 			f.UseVariable(variable)
-			return variable.Register, nil
+			return variable.Type, variable.Register, nil
 		}
 	}
 
 	tmp := f.NewRegister()
-	err := f.ExpressionToRegister(expr, tmp)
-	return tmp, err
+	typ, err := f.ExpressionToRegister(expr, tmp)
+	return typ, tmp, err
 }
diff --git a/src/build/core/Execute.go b/src/build/core/Execute.go
index 76a54bf..e4b7010 100644
--- a/src/build/core/Execute.go
+++ b/src/build/core/Execute.go
@@ -14,7 +14,7 @@ func (f *Function) Execute(operation token.Token, register cpu.Register, value *
 	}
 
 	if ast.IsFunctionCall(value) {
-		err := f.CompileCall(value)
+		_, err := f.CompileCall(value)
 
 		if err != nil {
 			return err
@@ -26,7 +26,7 @@ func (f *Function) Execute(operation token.Token, register cpu.Register, value *
 	tmp := f.NewRegister()
 	defer f.FreeRegister(tmp)
 
-	err := f.ExpressionToRegister(value, tmp)
+	_, err := f.ExpressionToRegister(value, tmp)
 
 	if err != nil {
 		return err
diff --git a/src/build/core/ExecuteLeaf.go b/src/build/core/ExecuteLeaf.go
index fe78ef6..2254a8a 100644
--- a/src/build/core/ExecuteLeaf.go
+++ b/src/build/core/ExecuteLeaf.go
@@ -31,7 +31,8 @@ func (f *Function) ExecuteLeaf(operation token.Token, register cpu.Register, ope
 
 	case token.String:
 		if operation.Kind == token.Assign {
-			return f.TokenToRegister(operand, register)
+			_, err := f.TokenToRegister(operand, register)
+			return err
 		}
 	}
 
diff --git a/src/build/core/ExpressionToMemory.go b/src/build/core/ExpressionToMemory.go
index 781b378..204e17d 100644
--- a/src/build/core/ExpressionToMemory.go
+++ b/src/build/core/ExpressionToMemory.go
@@ -5,30 +5,31 @@ import (
 	"git.akyoto.dev/cli/q/src/build/errors"
 	"git.akyoto.dev/cli/q/src/build/expression"
 	"git.akyoto.dev/cli/q/src/build/sizeof"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // ExpressionToMemory puts the result of an expression into the specified memory address.
-func (f *Function) ExpressionToMemory(node *expression.Expression, memory asm.Memory) error {
+func (f *Function) ExpressionToMemory(node *expression.Expression, memory asm.Memory) (types.Type, error) {
 	if node.IsLeaf() && node.Token.IsNumeric() {
 		number, err := f.Number(node.Token)
 
 		if err != nil {
-			return err
+			return types.Invalid, err
 		}
 
 		size := byte(sizeof.Signed(int64(number)))
 
 		if size != memory.Length {
-			return errors.New(&errors.NumberExceedsBounds{Number: number, ExpectedSize: memory.Length, Size: size}, f.File, node.Token.Position)
+			return types.Invalid, errors.New(&errors.NumberExceedsBounds{Number: number, ExpectedSize: memory.Length, Size: size}, f.File, node.Token.Position)
 		}
 
 		f.MemoryNumber(asm.STORE, memory, number)
-		return nil
+		return types.Int, nil
 	}
 
 	tmp := f.NewRegister()
 	defer f.FreeRegister(tmp)
-	err := f.ExpressionToRegister(node, tmp)
+	typ, err := f.ExpressionToRegister(node, tmp)
 	f.MemoryRegister(asm.STORE, memory, tmp)
-	return err
+	return typ, err
 }
diff --git a/src/build/core/ExpressionToRegister.go b/src/build/core/ExpressionToRegister.go
index 29a084c..c084351 100644
--- a/src/build/core/ExpressionToRegister.go
+++ b/src/build/core/ExpressionToRegister.go
@@ -7,15 +7,16 @@ import (
 	"git.akyoto.dev/cli/q/src/build/errors"
 	"git.akyoto.dev/cli/q/src/build/expression"
 	"git.akyoto.dev/cli/q/src/build/token"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // ExpressionToRegister puts the result of an expression into the specified register.
-func (f *Function) ExpressionToRegister(node *expression.Expression, register cpu.Register) error {
+func (f *Function) ExpressionToRegister(node *expression.Expression, register cpu.Register) (types.Type, error) {
 	f.SaveRegister(register)
 
 	if node.IsFolded {
 		f.RegisterNumber(asm.MOVE, register, node.Value)
-		return nil
+		return types.Int, nil
 	}
 
 	if node.IsLeaf() {
@@ -23,34 +24,38 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 	}
 
 	if ast.IsFunctionCall(node) {
-		err := f.CompileCall(node)
+		fn, err := f.CompileCall(node)
 
 		if register != f.CPU.Output[0] {
 			f.RegisterRegister(asm.MOVE, register, f.CPU.Output[0])
 		}
 
-		return err
+		if fn == nil || len(fn.ReturnTypes) == 0 {
+			return types.Invalid, err
+		}
+
+		return fn.ReturnTypes[0], err
 	}
 
 	if node.Token.Kind == token.Array {
 		array := f.VariableByName(node.Children[0].Token.Text(f.File.Bytes))
 		offset, err := f.Number(node.Children[1].Token)
 		f.MemoryRegister(asm.LOAD, asm.Memory{Base: array.Register, Offset: byte(offset), Length: 1}, register)
-		return err
+		return types.Int, err
 	}
 
 	if len(node.Children) == 1 {
 		if !node.Token.IsUnaryOperator() {
-			return errors.New(errors.MissingOperand, f.File, node.Token.End())
+			return types.Invalid, errors.New(errors.MissingOperand, f.File, node.Token.End())
 		}
 
-		err := f.ExpressionToRegister(node.Children[0], register)
+		typ, err := f.ExpressionToRegister(node.Children[0], register)
 
 		if err != nil {
-			return err
+			return typ, err
 		}
 
-		return f.ExecuteRegister(node.Token, register)
+		return typ, f.ExecuteRegister(node.Token, register)
 	}
 
 	left := node.Children[0]
@@ -61,10 +66,10 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		register = f.NewRegister()
 	}
 
-	err := f.ExpressionToRegister(left, register)
+	typ, err := f.ExpressionToRegister(left, register)
 
 	if err != nil {
-		return err
+		return types.Invalid, err
 	}
 
 	err = f.Execute(node.Token, register, right)
@@ -74,5 +79,5 @@ func (f *Function) ExpressionToRegister(node *expression.Expression, register cp
 		f.FreeRegister(register)
 	}
 
-	return err
+	return typ, err
 }
diff --git a/src/build/core/ExpressionsToRegisters.go b/src/build/core/ExpressionsToRegisters.go
index f3a09fe..982701a 100644
--- a/src/build/core/ExpressionsToRegisters.go
+++ b/src/build/core/ExpressionsToRegisters.go
@@ -8,7 +8,7 @@ import (
 // ExpressionsToRegisters moves multiple expressions into the specified registers.
 func (f *Function) ExpressionsToRegisters(expressions []*expression.Expression, registers []cpu.Register) error {
 	for i := len(expressions) - 1; i >= 0; i-- {
-		err := f.ExpressionToRegister(expressions[i], registers[i])
+		_, err := f.ExpressionToRegister(expressions[i], registers[i])
 
 		if err != nil {
 			return err
diff --git a/src/build/core/Function.go b/src/build/core/Function.go
index 9aeb10a..5f8c25c 100644
--- a/src/build/core/Function.go
+++ b/src/build/core/Function.go
@@ -3,21 +3,25 @@ package core
 import (
 	"git.akyoto.dev/cli/q/src/build/fs"
 	"git.akyoto.dev/cli/q/src/build/register"
+	"git.akyoto.dev/cli/q/src/build/scope"
 	"git.akyoto.dev/cli/q/src/build/token"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // Function represents the smallest unit of code.
 type Function struct {
 	register.Machine
-	Package    string
-	Name       string
-	UniqueName string
-	File       *fs.File
-	Body       []token.Token
-	Functions  map[string]*Function
-	Err        error
-	deferred   []func()
-	count      counter
+	Package     string
+	Name        string
+	UniqueName  string
+	File        *fs.File
+	Body        token.List
+	Parameters  []*scope.Variable
+	ReturnTypes []types.Type
+	Functions   map[string]*Function
+	Err         error
+	deferred    []func()
+	count       counter
 }
 
 // counter stores how often a certain statement appeared so we can generate a unique label from it.
diff --git a/src/build/core/TokenToRegister.go b/src/build/core/TokenToRegister.go
index 3335daf..c1d38ff 100644
--- a/src/build/core/TokenToRegister.go
+++ b/src/build/core/TokenToRegister.go
@@ -5,35 +5,36 @@ import (
 	"git.akyoto.dev/cli/q/src/build/cpu"
 	"git.akyoto.dev/cli/q/src/build/errors"
 	"git.akyoto.dev/cli/q/src/build/token"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // TokenToRegister moves a token into a register.
 // It only works with identifiers, numbers and strings.
-func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
+func (f *Function) TokenToRegister(t token.Token, register cpu.Register) (types.Type, error) {
 	switch t.Kind {
 	case token.Identifier:
 		name := t.Text(f.File.Bytes)
 		variable := f.VariableByName(name)
 
 		if variable == nil {
-			return errors.New(&errors.UnknownIdentifier{Name: name}, f.File, t.Position)
+			return types.Invalid, errors.New(&errors.UnknownIdentifier{Name: name}, f.File, t.Position)
 		}
 
 		f.UseVariable(variable)
 		f.SaveRegister(register)
 		f.RegisterRegister(asm.MOVE, register, variable.Register)
-		return nil
+		return variable.Type, nil
 
 	case token.Number, token.Rune:
 		number, err := f.Number(t)
 
 		if err != nil {
-			return err
+			return types.Invalid, err
 		}
 
 		f.SaveRegister(register)
 		f.RegisterNumber(asm.MOVE, register, number)
-		return nil
+		return types.Int, nil
 
 	case token.String:
 		data := t.Bytes(f.File.Bytes)
@@ -41,9 +42,9 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 		label := f.AddBytes(data)
 		f.SaveRegister(register)
 		f.RegisterLabel(asm.MOVE, register, label)
-		return nil
+		return types.Int, nil
 
 	default:
-		return errors.New(errors.InvalidExpression, f.File, t.Position)
+		return types.Invalid, errors.New(errors.InvalidExpression, f.File, t.Position)
 	}
 }
diff --git a/src/build/errors/Base.go b/src/build/errors/Base.go
index 136c9bf..0933e59 100644
--- a/src/build/errors/Base.go
+++ b/src/build/errors/Base.go
@@ -19,6 +19,7 @@ var (
 	MissingOperand             = &Base{"Missing operand"}
 	MissingType                = &Base{"Missing type"}
 	NotImplemented             = &Base{"Not implemented"}
+	UnknownType                = &Base{"Unknown type"}
 )
 
 // Base is the base class for errors that have no parameters.
diff --git a/src/build/scanner/scanFile.go b/src/build/scanner/scanFile.go
index 59260bd..b7174b0 100644
--- a/src/build/scanner/scanFile.go
+++ b/src/build/scanner/scanFile.go
@@ -1,7 +1,6 @@
 package scanner
 
 import (
-	"fmt"
 	"os"
 	"path/filepath"
 
@@ -13,6 +12,7 @@ import (
 	"git.akyoto.dev/cli/q/src/build/fs"
 	"git.akyoto.dev/cli/q/src/build/scope"
 	"git.akyoto.dev/cli/q/src/build/token"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // scanFile scans a single file.
@@ -42,6 +42,8 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 		paramsStart = -1
 		paramsEnd   = -1
 		bodyStart   = -1
+		typeStart   = -1
+		typeEnd     = -1
 	)
 
 	for {
@@ -156,9 +158,13 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 
 		// Return type
 		if i < len(tokens) && tokens[i].Kind == token.ReturnType {
+			typeStart = i + 1
+
 			for i < len(tokens) && tokens[i].Kind != token.BlockStart {
 				i++
 			}
+
+			typeEnd = i
 		}
 
 		// Function definition
@@ -221,6 +227,11 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 		name := tokens[nameStart].Text(contents)
 		body := tokens[bodyStart:i]
 		function := core.NewFunction(pkg, name, file, body)
+
+		if typeStart != -1 {
+			function.ReturnTypes = append(function.ReturnTypes, types.New(tokens[typeStart:typeEnd]))
+		}
+
 		parameters := tokens[paramsStart:paramsEnd]
 		count := 0
 
@@ -230,8 +241,7 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 			}
 
 			name := tokens[0].Text(contents)
-			dataType := tokens[1].Text(contents)
-			fmt.Println(dataType)
+			dataType := types.New(tokens[1:])
 			register := x64.CallRegisters[count]
 			uses := token.Count(function.Body, contents, token.Identifier, name)
 
@@ -241,10 +251,12 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 
 			variable := &scope.Variable{
 				Name:     name,
+				Type:     dataType,
 				Register: register,
 				Alive:    uses,
 			}
 
+			function.Parameters = append(function.Parameters, variable)
 			function.AddVariable(variable)
 			count++
 			return nil
@@ -258,6 +270,8 @@ func (s *Scanner) scanFile(path string, pkg string) error {
 		nameStart = -1
 		paramsStart = -1
 		bodyStart = -1
+		typeStart = -1
+		typeEnd = -1
 		i++
 	}
 }
diff --git a/src/build/scope/Variable.go b/src/build/scope/Variable.go
index 491bb78..0346ca1 100644
--- a/src/build/scope/Variable.go
+++ b/src/build/scope/Variable.go
@@ -2,11 +2,13 @@ package scope
 
 import (
 	"git.akyoto.dev/cli/q/src/build/cpu"
+	"git.akyoto.dev/cli/q/src/build/types"
 )
 
 // Variable represents a named register.
 type Variable struct {
 	Name     string
+	Type     types.Type
 	Alive    uint8
 	Register cpu.Register
 }
diff --git a/src/build/types/New.go b/src/build/types/New.go
new file mode 100644
index 0000000..73e7d23
--- /dev/null
+++ b/src/build/types/New.go
@@ -0,0 +1,8 @@
+package types
+
+import "git.akyoto.dev/cli/q/src/build/token"
+
+// New creates a new type from a list of tokens.
+func New(tokens token.List) Type {
+	return Int
+}
diff --git a/src/build/types/Type.go b/src/build/types/Type.go
new file mode 100644
index 0000000..07340a0
--- /dev/null
+++ b/src/build/types/Type.go
@@ -0,0 +1,8 @@
+package types
+
+type Type int
+
+const (
+	Invalid Type = iota // Invalid is an invalid type.
+	Int
+)