]> Antoine Delignat-Lavaud wrote: > I chose to solve the problem of polymorphic references by adding value > restriction* to my inferer, using ocaml to check my results. > Not knowing whether the empty list should be considered a value or an > expression, I copied Ocaml's behavior and made it a value. Yes, the empty list is a value, like all other constants. > As a result, my inferer gave the following expression the integer type : > let el = [] in if hd el then 1 else hd el ;; > which is the expected result since el has polymorphic type 'a list > but does not look right because it is used as both a bool list and an > int list. It is perfectly right. The empty list can of course be used both as a bool list and an int list; that's exactly what parametric polymorphism is all about. Richard Jones wrote: > But the same if statement within a function definition causes an error: > > # let f el = > if List.length el > 0 then (List.hd el)+(int_of_string (List.hd el)) else 0;; > ^^^^^^^^^^ > This expression has type int but is here used with type string This is Hindley-Milner polymorphism at work: only "let"-bound variables can have polymorphic types, while function parameters are monomorphic. - Xavier Leroy