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Date: | 2002-08-18 (16:13) |
From: | John Max Skaller <skaller@o...> |
Subject: | Re: [Caml-list] Cyclic ?! |
Oleg wrote: > Hi > > I'm puzzled by the following compiler behavior: > > If I define bin_tree as > > type 'a bin_tree = > Empty > | Node of 'a bin_tree * 'a * 'a bin_tree > > the compiler accepts it. OTOH if I define it as > > type 'a bin_tree = ('a bin_tree * 'a * 'a bin_tree) option > > It gives an error: "The type abbreviation bin_tree is cyclic". > Why??? And what's the difference between the two, really? No semantic difference at all. But there is a syntactic difference: ocaml (without -rectypes) only allows cycles across "object boundaries", that includes across variant constructor names. In the 'option' case, the constructor name in 'option' type (namely "Some") isn't explicitly separating the LHS and RHS parts of the declaration. The syntax checker doesn't know what 'option' is yet, it's just an arbitrary polymorphic type: it could be: type 'a option = 'a in which case, there really is an error. Note that a polymorphic type (like option) could be abstract -- in which case we don't know it's internals, and there is no choice but to reject the above style in that case (assuming we want to enforce the 'no recursion except across object boundaries' rule). So it seems the check is done *before* the lookup for the meaning of 'option' which is why I called it a 'syntax check' -- it is based entirely on the shape of the declaration in the abstract -- without considering the meaning of the subterms. This makes the rule robust in the face of substitution (changing 'option' to 'id' for example, where 'a id = 'a) -- 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