0005724: Interaction between GADTs and polymorphic variants

Notes
(0007940) garrigue (manager) 2012-08-14 03:01	This is a documented limitation of GADTs: types cannot be refined if a pattern-matching contains polymorphic variant. This comes from the fact polymorphic variant pattern-matching typing is specified in the absence of type propagation and GADT type refinement requires type propagation. This might be improved in the future, but there is no complete solution.

(0008808) yallop (reporter) 2013-01-28 13:21	It's good to see that the trunk typechecker now accepts the example.

(0008810) garrigue (manager) 2013-01-29 01:26	Re-open before changing resolution.

(0008811) garrigue (manager) 2013-01-29 01:28	This is now fixed in trunk, since revision 13221. Here is the corresponding message sent to the caml-list: As you may be aware from past threads, since the introduction of GADTs in 4.00, some type information is propagated to pattern-matching, to allow it to refine types. More recently, types have started being used to disambiguate constructors and record fields, which means some more dependency on type information in pattern-matching. However, a weakness of this approach was that propagation was disabled as soon as a pattern contained polymorphic variants. The reason is that typing rules for polymorphic variants in patterns and expression are subtly different, and mixing information without care would lose principality. At long last I have removed this restriction on the presence of polymorphic variants, but this has some consequences on typing: * while type information is now propagated, information about possibly present constructors still has to be discarded. For instance this means that the following code will not be typed as you could expect: let f (x : [< `A \| `B]) = match x with `A -> 1 \| _ -> 2;; val f : [< `A \| `B > `A ] -> int What happens is that inside pattern-matching, only required constructors are propagated, which reduces the type of x to [> ] (a polymorphic variant type with any constructor…) As before, to give an upper bound to the matched type, the type annotation must be inside a pattern: let f = function (`A : [< `A \| `B]) -> 1 \| _ -> 2;; val f : [< `A \| `B ] -> int = <fun> * the propagation of type information may lead to failure in some cases that where typable before: type ab = [ `A \| `B ];; let f (x : [`A]) = match x with #ab -> 1;; Error: This pattern matches values of type [? `A \| `B ] but a pattern was expected which matches values of type [ `A ] The second variant type does not allow tag(s) `B During pattern-matching it is not allowed to match on absent type constructors, even though the type of the patterns would eventually be [< `A \| `B], which allows discarding `B. ([? `A \| `B] denotes a type obeying the rules of pattern-matching) * for the sake of coherence, even if a type was not propagated because it was not known when typing a pattern-matching, we are still going to fail if a matched constructor appears to be absent after typing the whole function. (This only applies to propagable types, i.e. polymorphic variant types that contain only required constructors) In particular the last two points are important, because previously such uses would not have triggered even a warning. The idea is that allowing propagation of types is more important than keeping some not really useful corner cases, but if this is of concern to you, I'm interested in feedback.

Issue History
Date Modified	Username	Field	Change
2012-08-14 02:34	yallop	New Issue
2012-08-14 03:01	garrigue	Note Added: 0007940
2012-08-14 03:01	garrigue	Status	new => resolved
2012-08-14 03:01	garrigue	Resolution	open => not fixable
2012-08-14 03:01	garrigue	Assigned To	=> garrigue
2013-01-28 13:21	yallop	Note Added: 0008808
2013-01-29 01:26	garrigue	Note Added: 0008810
2013-01-29 01:26	garrigue	Status	resolved => confirmed
2013-01-29 01:28	garrigue	Note Added: 0008811
2013-01-29 01:28	garrigue	Status	confirmed => resolved
2013-01-29 01:28	garrigue	Fixed in Version	=> 4.01.0+dev
2013-01-29 01:28	garrigue	Resolution	not fixable => fixed
2013-04-23 02:40	garrigue	Relationship added	child of 0005998