Zheng Li <zheng...@users.sourceforge.net> writes:
> [OT]: Here are a few more annoyances on the syntax of instance
> variable I've encountered. I recorded it here for comments:
>
> Why it can't just copy the common "let" syntax in OCaml? Even though
> it's possible to just lift the awkward "val" definitions out of
> objects and using "let" instead, I'm really uncomfortable when being
> forced to do that. Besides, the lifted variables won't be accessible
> through inheritance any more, so they are not strictly equivalent or
> interchangeable in semantics.
>
> Currently, the "val" syntax
>
> - doesn't support pattern matching assignment. Instead of
> --
> let a,b,c,d = tuple4
> --
> one write
> --
> val a = match tuple4 with a,_,_,_ -> a
> val b = match tuple4 with _,b,_,_ -> b
> val c = match tuple4 with _,_,c,_ -> c
> val d = match tuple4 with _,_,_,d -> d
> --
> This is still the easy case, image if your pattern matching will
> also trigger some side-effect.
class foo =
let (a,b,c,d) = tuple4
in
object
val a = a val b = b val c = c val d = d
end
> - doesn't support sharing state.
> --
> let up,down =
> let r = ref 0 in
> (fun () -> incr r; !r),
> (fun () -> decr r; !r)
> --
> can't be expressed directly as instance variables
Same thing. Use let to create the shared state and then assign in the object.
Although that really ought to be written as
class foo : object (* to hide count if you really want that *)
method private up : unit
method private down : unit
end = object
val count = ref 0
method private up = incr count
method private down = decr count
end
> - doesn't support shortcut syntax of function definition like
> --
> val f x y z = x + y + z
> --
> instead one must write
> --
> val f = fun x y z -> x + y + z
> --
same
> - doesn't support recursion. Instead of
> --
> val rec fact n =
> if n <= 1 then 1 else n * fact (n-1)
> --
> one must write
> --
> val fact =
> let rec fact_rec n =
> if n <= 1 then 1 else n * fact_rec (n-1) in
> fact_rec
> --
same
> - all parallel assignments, no assignment orders, hence can't refer to
> previous assignment
>
> --
> val x = 3
> val y = x + 11
> --
>
> is wrong, even if it makes perfect sense in many situations. To access
> "x", one is simply forced to declare "y" as method!
same again.
> Any comments?
Yes, ocaml objects can drive one nuts. :)
You seem to want to use classes with static functions (functions that
do not have/use self). Why not use records? Is that just because you
can not coerce record to a "subtype"? Sometimes one can use modules
and functors instead of objects.
MfG
Goswin
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