From: Francois Rouaix <francois@rouaix.org>

> Hi all,
> The following example mystifies me
> 
> let f ?opt x =
>    match opt with
>    | None -> x
>    | Some dx -> x + dx
> 
> let g x = x + 1
> 
> 
> let h1 = function
>    | 0 -> [g]
>    | _ -> [f; g]
> 
> 
> let h2 = function
>    | _ -> [f; g]
> 
> 
> h1 is typable but not h2.
> Why is that ?

As stated in many places in the manual, type inference for optional
arguments is not principal.
But there is a now a -principal option to ocaml and ocamlc, which
attempts to recover principality, and gives more information in such
cases.

% ocaml -principal
        Objective Caml version 3.06+27 (2003-03-31)

[...]
# let h1 = function
     | 0 -> [g]
     | _ -> [f; g]
             ^
  ;;
Warning: Eliminated optional argument without principality
val h1 : int -> (int -> int) list = <fun>


Note that -principal itself is available in ocaml 3.06, but would not
provide the above warning.

Jacques Garrigue

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