diff --git a/src/build/Execute.go b/src/build/Execute.go
index bd50435..3bdc61e 100644
--- a/src/build/Execute.go
+++ b/src/build/Execute.go
@@ -190,6 +190,16 @@ func (f *Function) TokenToRegister(t token.Token, register cpu.Register) error {
 	switch t.Kind {
 	case token.Identifier:
 		name := t.Text()
+		constant, exists := f.definitions[name]
+
+		if exists {
+			if config.Verbose {
+				f.Logf("constant %s = %s", constant.Name, constant.Value)
+			}
+
+			return f.ExpressionToRegister(constant.Value, register)
+		}
+
 		variable, exists := f.variables[name]
 
 		if !exists {
diff --git a/src/build/Function.go b/src/build/Function.go
index 8853605..d58afc2 100644
--- a/src/build/Function.go
+++ b/src/build/Function.go
@@ -133,6 +133,18 @@ func (f *Function) Wait() {
 // identifierExists returns true if the identifier has been defined.
 func (f *Function) identifierExists(name string) bool {
 	_, exists := f.variables[name]
+
+	if exists {
+		return true
+	}
+
+	_, exists = f.definitions[name]
+
+	if exists {
+		return true
+	}
+
+	_, exists = f.functions[name]
 	return exists
 }
 
diff --git a/src/build/Variable.go b/src/build/Variable.go
index 9d18c37..291d2d4 100644
--- a/src/build/Variable.go
+++ b/src/build/Variable.go
@@ -2,11 +2,18 @@ package build
 
 import (
 	"git.akyoto.dev/cli/q/src/build/cpu"
+	"git.akyoto.dev/cli/q/src/build/expression"
 )
 
-// Variable represents a variable in a function.
+// Variable represents a named register.
 type Variable struct {
 	Name     string
 	Register cpu.Register
 	Alive    int
 }
+
+// Definitions are single use expressions that don't reside in a register yet.
+type Definition struct {
+	Name  string
+	Value *expression.Expression
+}
diff --git a/src/build/VariableDefinition.go b/src/build/VariableDefinition.go
index f0624da..58a4844 100644
--- a/src/build/VariableDefinition.go
+++ b/src/build/VariableDefinition.go
@@ -25,25 +25,32 @@ func (f *Function) CompileVariableDefinition(expr *expression.Expression) error
 		return errors.New(&errors.UnusedVariable{Name: name}, f.File, expr.Children[0].Token.Position)
 	}
 
-	reg, exists := f.cpu.FindFree(f.cpu.General)
+	value := expr.Children[1]
 
-	if !exists {
-		panic("no free registers")
-	}
+	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)
+		}
 
-	err := f.ExpressionToRegister(expr.Children[1], reg)
+		return nil
+	})
 
 	if err != nil {
 		return err
 	}
 
-	f.addVariable(&Variable{
-		Name:     name,
-		Register: reg,
-		Alive:    uses,
-	})
+	if uses == 1 {
+		expr.RemoveChild(value)
 
-	return nil
+		f.definitions[name] = &Definition{
+			Name:  name,
+			Value: value,
+		}
+
+		return nil
+	}
+
+	return f.storeVariableInRegister(name, value, uses)
 }
 
 func (f *Function) addVariable(variable *Variable) {
@@ -75,6 +82,24 @@ func (f *Function) useVariable(variable *Variable) {
 	}
 }
 
+func (f *Function) storeVariableInRegister(name string, value *expression.Expression, uses int) error {
+	reg, exists := f.cpu.FindFree(f.cpu.General)
+
+	if !exists {
+		panic("no free registers")
+	}
+
+	err := f.ExpressionToRegister(value, reg)
+
+	f.addVariable(&Variable{
+		Name:     name,
+		Register: reg,
+		Alive:    uses,
+	})
+
+	return err
+}
+
 func countIdentifier(tokens token.List, name string) int {
 	count := 0
 
diff --git a/src/build/compiler.go b/src/build/compiler.go
index 19b75a2..60cfec0 100644
--- a/src/build/compiler.go
+++ b/src/build/compiler.go
@@ -16,6 +16,7 @@ type compiler struct {
 	cpu         cpu.CPU
 	debug       []debug
 	err         error
+	definitions map[string]*Definition
 	variables   map[string]*Variable
 	functions   map[string]*Function
 	sideEffects int
diff --git a/src/build/scan.go b/src/build/scan.go
index 701a824..758bbec 100644
--- a/src/build/scan.go
+++ b/src/build/scan.go
@@ -250,8 +250,9 @@ func scanFile(path string, functions chan<- *Function) error {
 					Syscall: x64.SyscallRegisters,
 					Return:  x64.ReturnValueRegisters,
 				},
-				variables: map[string]*Variable{},
-				finished:  make(chan struct{}),
+				definitions: map[string]*Definition{},
+				variables:   map[string]*Variable{},
+				finished:    make(chan struct{}),
 			},
 		}