"Damien Guichard" <alphabl...@orange.fr> writes:
>> I once faced this situation and the solution is to use modules.
> That is one good practical solution.
>
> The simpler solution that immediatly came to my mind is eta-expansion.
>
> type foo = {bar : 'a. 'a -> 'a}
> let a : int -> int = fun x -> x
> let baz = {bar = fun x -> (Obj.magic a) x}
This would create a new closure though. Wastes time and space. Bad if
you create a million foo's all with the same function.
> However it issues a warning so i acknowledge it's less elegant.
Which I don't quite understand.
> - damien
Why not use magic on the overall type of the record instead of the
individual function?
# type foo = { foo : 'a. 'a -> 'a; }
type 'a bar = {bar : 'a -> 'a}
let a : int -> int = fun x -> x
let baz = (Obj.magic {bar = a} : foo);;
type foo = { foo : 'a. 'a -> 'a; }
type 'a bar = { bar : 'a -> 'a; }
val a : int -> int = <fun>
val baz : foo = {foo = <fun>}
It means duplicating the record type but you can magic it without
warnings.
Overall I have to say though that you are doing something wrong here.
(Should I assume your actual use case is larger than this simple
example?)
# let b x = x + 1
let buzz = (Obj.magic {bar = b} : foo)
buzz.foo [1;2;3];;
val b : int -> int = <fun>
val buzz : foo = {foo = <fun>}
- : int list = [-2330612807164231680]
Doesn't always segfault but lets get nastier:
# (buzz.foo buzz).foo 1;;
zsh: segmentation fault ocaml
The obj.magic only works safely if the function you use is of type 'a
-> 'a (even if the interface restricts it to a less general type). The
right solution would be to loosen the interface to use the more
general type.
MfG
Goswin
_______________________________________________
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
