Unbound values not making sense

For some reason, I get that raise and = (for the id comparison) are unbound.

type id = stringlet rec lookup (id: id) (storage: (id * 'a) list): 'a =  match storage with  | [] -> raise Not_found  | (id', element)::storage' ->    if id = id'    then element    else lookup id storage'

Also, in this case where value1 and value2 have the type int, I get that + is unbound.

let rec interpretExpression  (expression: expression)  (valueStorage: value_storage)  (functionStorage: function_storage): int =  match expression with  | Id id -> lookup id valueStorage  | Int value -> value  | Add (expression1, expression2) ->    let value1 = interpretExpression expression1 valueStorage functionStorage    and value2 = interpretExpression expression2 valueStorage functionStorage    in value1 + value2  | FuncApp id ->    let expression' = lookup id functionStorage    in interpretExpression expression' valueStorage functionStorage

I found many other posts about unbound values for OCaml, but they were about

• Not having evaluated bindings
• Forgetting to use rec for recursive functions
• Scoping

but raise, =, and + are all part of the core language.

Why I get unbound value for these operations?

Edit: