More specific explanation of an error in OCaml

Question

In the ensemble.mli file I have

type 'a t;;
val ajouter : 'a -> 'a t -> 'a t 

And in the ensemble.ml file:

type 'a t='a list
let rec ajouter =function a -> function 
     [] ->[a]
     |h::t -> if h=a then h::t else h::(ajouter a t) 

After compilation:

ocamlc ensemble.mli ensemble.ml

#load "ensemble.cmo"
Ensemble.ajouter 1 [];;

**Error: This expression has type 'a list but an expression 
   was expected of type int Ensemble.t**

I have to be able to give a detailed explanation about this error but I don't know exactly how to even fix it.So I really need your help.

Answer 1

Your type t is abstract. So nothing that comes from outside the Ensemble module can have that type. Thus, [] doesn't have the type 'a Ensemble.t as you would wish.

The usual solution is to export a value named (say) empty (or vide) from the module.