Interesting behavior of "include"
 John Prevost
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Date:  19990129 (08:51) 
From:  John Prevost <prevost@m...> 
Subject:  Interesting behavior of "include" 
I started using the "include" directive in signatures recently, then realized that it's not actually documented anywhere (although it's quite useful at times.) I also discovered a little "problem" with it. module type test = sig type t val a : t end module type test2 = sig include test val b : t end module type test3 = sig include test val c : t end module type testbad = sig include test2 include test3 with type t = t end (* This would seem to imply that the new type t is the same as the old when the above expands to: sig type t val a : t val b : t type t = t (* just "type t" without the "with" part above. *) val a : t val c : t end So that a, b, and c would all have the same type. If you try it without the with, you'll see that they do actually end up all with the same type, but no matter how hard you try to constrain the signature using "with", the second "type t" causes t to be abstract. If you use the with above to try to constrain the second type t in the signature to be the same as the first (you can't name which t you mean, since include doesn't let you refer to the first t as Test2.t), you get an even worse problem: module type testbad = sig type t val a : t val b : t type t = t val a : t val c : t end # module Testbad : testbad with type t = int = struct # type t = int # let a = 1 # let b = 1 # let c = 1 # end;; module Testbad : sig type t = int val a : t val b : t type t = t val a : t val c : t end # Testbad.a;;  : Testbad.t = Uncaught exception: Stack overflow This happens with or without the with in the module definition. Pretty neat, eh? The with definition is allowing you to define a cyclic type abbreviation. (This is correctly caught in all other places you can use with as being an unbound type constructor. But in the include case, it must see that there is, in fact, a type t, without being able to express in the form of a signature that in the definition "type t = t", the second t refers to the previously defined t. This leaves two problems of course: the first is "include ... with" needs to check for cyclic type abbreviations, and the second is that there's no way to constrain includes to be more "merging" than they are (so that I can define "MONAD_ZERO_PLUS" in terms of "MONAD_ZERO" and "MONAD_PLUS" instead of in terms of "MONAD", say.) Of course, I probably shouldn't be using an undocumented feature in the first place. *) jmp