On 2010/09/24, at 0:05, Adrien wrote: > On 23/09/2010, bluestorm <bluestorm.d...@gmail.com> wrote: >> (sorry for any double-posting) >> >> The problem is that in your declaration of h, the inferred type for f >> is of the form (unit -> unit -> ...), and you use it with the >> different type (unit -> ?a:'a -> unit -> ...). >> >> Changing ?a to be the first parameter of f change f's type to (?a:'a >> -> unit -> unit -> ...). OCaml knows that it can implicitly coerce >> functions when the optional parameters appear in the first position. > > I actually expected types (?a:'a -> unit -> unit -> ...) and (unit -> > ?a:'a -> unit -> ...) to work alike. I'm wondering why the arguments > are "dropped" from the first case but not the second.
The answer is that this way of coercing functions by applying optional arguments when they are passed is a kind of hack. The formal semantics defines optional argument discarding as happening only when a function is applied. However, one often wants this discarding to occur when the function is passed as argument, to avoid manual eta-expansion. This is actually done through an eta-expansion (i.e., (fun x -> f x) gets passed to the function instead of f, to be sure that side-effects happen at the right time). Since only cases with a single eta-expansion are considered, optional arguments should come first. Of course we are very careful of not breaking the formal semantics (where there is no eta-expansion, and discarding will only occur when the function is applied). Whenever the eta-expansion approach might introduce a discrepancy, you will get a type error. Jacques Garrigue _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
