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Date: | 2002-08-23 (22:38) |
From: | John Max Skaller <skaller@o...> |
Subject: | Re: [Caml-list] Naming polymorphic variant types |
Nick Alexander wrote: > Heh, that's what I thought. Oh well... maybe there is a way to do what I > want anyway. Let's say I have a record, say {a:int; b:int; c:int}. I > wish to have a function that returns sub-records, say {a:int; b:int} or > {b:int; c:int}. However, I don't want to (and can't, if I recall, due to > name clashes) declare all the sub record types. > I certainly don't understand enough type/category theory to > extend the system myself. > > Am I out of luck? AFAIK there's no reason it can't be implemented, but the result would not be compatible with the existing system. For example: type t = {a:int; b:int} let f x = x.a + 1 The type has to be assumed to be {>a:int} -> int (* #1 *) or {a:int} -> int (* #2 *) with polymorphic records (the declaration of type t is irrelevant). But the existing system deduces t -> int For polymorphic variants, a new syntax was used to get around the equivalent problem: that seems to be the main obstacle to me. The obvious candidate is: let f x = x.`a just like polymorphic variants. `a means polymorphic record label after a . or inside {} in leading position (after '{' or ';') It's not clear whether this style, with automatic conversions (#1): let f x = x.`a in ignore(f {`a=1}); ignore(f {`a=1; `b=2}) (* #1 *) would be better than requiring explicit conversion (* #2 *) .. f( {`a=1; `b=2) :> {`a:int} ) )(* #2 *) .. but clearly a physical conversion (actual run time operation) is required, so that the field is at the right offset. With polymorphic variants no physical conversion is required, it's a compile time typing thing only. [The typing in presence of mutable fields may complicate things] -- John Max Skaller, mailto:skaller@ozemail.com.au snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia. voice:61-2-9660-0850 ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners