```
data Type = Nat | Bool | App Type Type | Var String 
     deriving (Eq, Show)

type Substitution = [(String, Type)]

apply :: Substitution -> Type -> Type
apply s Nat = Nat
apply s Bool = Bool
apply s Var c = case (lookup s c) of 
                Nothing -> (Var c)
                Just v  ->  v

```

But the compilers give me the error "error: parse error on input ‘Just’
"

What am I doing wrong?