diff --git a/src/build/core/CompileCall.go b/src/build/core/CompileCall.go
index 079cf97..9c92950 100644
--- a/src/build/core/CompileCall.go
+++ b/src/build/core/CompileCall.go
@@ -2,6 +2,7 @@ package core
 
 import (
 	"git.akyoto.dev/cli/q/src/build/asm"
+	"git.akyoto.dev/cli/q/src/build/errors"
 	"git.akyoto.dev/cli/q/src/build/expression"
 )
 
@@ -11,9 +12,18 @@ import (
 // After the function call, they must be restored in reverse order.
 func (f *Function) CompileCall(root *expression.Expression) error {
 	funcName := root.Children[0].Token.Text()
+	isSyscall := funcName == "syscall"
+
+	if !isSyscall {
+		_, exists := f.functions[funcName]
+
+		if !exists {
+			return errors.New(&errors.UnknownFunction{Name: funcName}, f.File, root.Children[0].Token.Position)
+		}
+	}
+
 	parameters := root.Children[1:]
 	registers := f.cpu.Input[:len(parameters)]
-	isSyscall := funcName == "syscall"
 
 	if isSyscall {
 		registers = f.cpu.Syscall[:len(parameters)]
diff --git a/src/build/core/CompileIf.go b/src/build/core/CompileIf.go
index bebfc10..de43bec 100644
--- a/src/build/core/CompileIf.go
+++ b/src/build/core/CompileIf.go
@@ -9,25 +9,15 @@ import (
 
 // CompileIf compiles a branch instruction.
 func (f *Function) CompileIf(branch *ast.If) error {
-	condition := branch.Condition
-	tmp := f.cpu.MustFindFree(f.cpu.General)
-	err := f.ExpressionToRegister(condition.Children[0], tmp)
+	err := f.Evaluate(branch.Condition)
 
 	if err != nil {
 		return err
 	}
 
-	f.cpu.Use(tmp)
-	err = f.Execute(condition.Token, tmp, condition.Children[1])
-
-	if err != nil {
-		return err
-	}
-
-	f.cpu.Free(tmp)
 	endLabel := fmt.Sprintf("%s_end_if_%d", f.Name, f.count.branch)
 
-	switch condition.Token.Text() {
+	switch branch.Condition.Token.Text() {
 	case "==":
 		f.assembler.Label(asm.JNE, endLabel)
 	case "!=":
diff --git a/src/build/core/Evaluate.go b/src/build/core/Evaluate.go
new file mode 100644
index 0000000..5c3a60c
--- /dev/null
+++ b/src/build/core/Evaluate.go
@@ -0,0 +1,41 @@
+package core
+
+import (
+	"git.akyoto.dev/cli/q/src/build/ast"
+	"git.akyoto.dev/cli/q/src/build/expression"
+	"git.akyoto.dev/cli/q/src/build/token"
+)
+
+// Evaluate evaluates an expression.
+func (f *Function) Evaluate(value *expression.Expression) error {
+	left := value.Children[0]
+	right := value.Children[1]
+
+	if left.IsLeaf() && left.Token.Kind == token.Identifier {
+		variable := f.variables[left.Token.Text()]
+		register := variable.Register
+		defer f.useVariable(variable)
+		return f.Execute(value.Token, register, right)
+	}
+
+	if ast.IsFunctionCall(left) && right.IsLeaf() {
+		err := f.CompileCall(left)
+
+		if err != nil {
+			return err
+		}
+
+		return f.Execute(value.Token, f.cpu.Output[0], right)
+	}
+
+	tmp := f.cpu.MustFindFree(f.cpu.General)
+	err := f.ExpressionToRegister(left, tmp)
+
+	if err != nil {
+		return err
+	}
+
+	f.cpu.Use(tmp)
+	defer f.cpu.Free(tmp)
+	return f.Execute(value.Token, tmp, right)
+}
diff --git a/src/build/errors/UnknownFunction.go b/src/build/errors/UnknownFunction.go
new file mode 100644
index 0000000..a2b7e4e
--- /dev/null
+++ b/src/build/errors/UnknownFunction.go
@@ -0,0 +1,18 @@
+package errors
+
+import "fmt"
+
+// UnknownFunction represents unknown function errors.
+type UnknownFunction struct {
+	Name        string
+	CorrectName string
+}
+
+// Error generates the string representation.
+func (err *UnknownFunction) Error() string {
+	if err.CorrectName != "" {
+		return fmt.Sprintf("Unknown function '%s', did you mean '%s'?", err.Name, err.CorrectName)
+	}
+
+	return fmt.Sprintf("Unknown function '%s'", err.Name)
+}
diff --git a/tests/errors/UnknownFunction.q b/tests/errors/UnknownFunction.q
new file mode 100644
index 0000000..6b917e1
--- /dev/null
+++ b/tests/errors/UnknownFunction.q
@@ -0,0 +1,3 @@
+main() {
+	unknown()
+}
\ No newline at end of file
diff --git a/tests/errors_test.go b/tests/errors_test.go
index d1123bb..7782a67 100644
--- a/tests/errors_test.go
+++ b/tests/errors_test.go
@@ -33,6 +33,7 @@ var errs = []struct {
 	{"MissingOperand.q", errors.MissingOperand},
 	{"MissingOperand2.q", errors.MissingOperand},
 	{"VariableAlreadyExists.q", &errors.VariableAlreadyExists{Name: "x"}},
+	{"UnknownFunction.q", &errors.UnknownFunction{Name: "unknown"}},
 	{"UnknownIdentifier.q", &errors.UnknownIdentifier{Name: "x"}},
 	{"UnknownIdentifier2.q", &errors.UnknownIdentifier{Name: "x"}},
 	{"UnknownIdentifier3.q", &errors.UnknownIdentifier{Name: "f"}},
diff --git a/tests/programs/branch.q b/tests/programs/branch.q
new file mode 100644
index 0000000..547fcfc
--- /dev/null
+++ b/tests/programs/branch.q
@@ -0,0 +1,81 @@
+main() {
+	x := 0
+
+	if x != 0 {
+		exit(1)
+	}
+
+	if x > 0 {
+		exit(1)
+	}
+
+	if x < 0 {
+		exit(1)
+	}
+
+	if 0 != x {
+		exit(1)
+	}
+
+	if 0 > x {
+		exit(1)
+	}
+
+	if 0 < x {
+		exit(1)
+	}
+
+	if x >= 1 {
+		exit(1)
+	}
+
+	if 1 <= x {
+		exit(1)
+	}
+
+	if x == inc(x) {
+		exit(1)
+	}
+
+	if x == dec(x) {
+		exit(1)
+	}
+
+	if inc(0) == x {
+		exit(1)
+	}
+
+	if dec(0) == x {
+		exit(1)
+	}
+
+	if inc(x) == dec(x) {
+		exit(1)
+	}
+
+	if x + 1 != inc(x) {
+		exit(1)
+	}
+
+	if x + 1 != x + 1 {
+		exit(1)
+	}
+
+	if x == 0 {
+		exit(0)
+	}
+	
+	exit(1)
+}
+
+exit(x) {
+	syscall(60, x)
+}
+
+inc(x) {
+	return x + 1
+}
+
+dec(x) {
+	return x - 1
+}
\ No newline at end of file