[Caml-list] Error: In the definition of t, type ('a, 'b) t should be ('b, 'a) t
This error message was new to me, and I wondered what's going on and why: # type ('a, 'b) t = [ `A | `T of ('b, 'a) t ];; Error: In the definition of t, type ('a, 'b) t should be ('b, 'a) t I can get unhelpful suggestions through foolishness with three type parameters: # type ('a, 'b, 'c) t = [ `A | `T of ('a, 'c, 'b) t | `V of ('b, 'c, 'a) t];; Error: In the definition of t, type ('a, 'b, 'c) t should be ('c, 'a, 'b) t # type ('c, 'a, 'b) t = [ `A | `T of ('a, 'c, 'b) t | `V of ('b, 'c, 'a) t];; Error: In the definition of t, type ('a, 'b, 'c) t should be ('b, 'c, 'a) t Any explanation of what's going on? Thanks, E ___ 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
Re: [Caml-list] Error: In the definition of t, type ('a, 'b) t should be ('b, 'a) t
From: Edgar Friendly thelema...@gmail.com This error message was new to me, and I wondered what's going on and why: # type ('a, 'b) t = [ `A | `T of ('b, 'a) t ];; Error: In the definition of t, type ('a, 'b) t should be ('b, 'a) t Structural recursive types (objects and polymorphic variants) must be regular. I.e., type abbreviations must always have the same parameters. The error message is a bit confusing, due to the way type variables are printed (original names are not kept), but it says that type parameters got exchanged. If you really want to define this type, you have to unroll it by hand: type ('a, 'b) t = [ `A of 'a | `T of [ `A of 'b | `T of ('a,'b) t]];; (I added an argument to `A to make the parameters meaningful.) 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