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|ID||Project||Category||View Status||Date Submitted||Last Update|
|0005724||OCaml||OCaml general||public||2012-08-14 02:34||2015-12-11 19:18|
|Target Version||Fixed in Version||4.01.0+dev|
|Summary||0005724: Interaction between GADTs and polymorphic variants|
|Description||OCaml reports an error for the last line of the following program:|
type _ t =
| A : int t
| B : [`C] -> bool t
let isA (type a) (r : a t) = match r with
| A -> true
| B `C -> false
Error: This pattern matches values of type bool t
but a pattern was expected which matches values of type int t
If the `C in the pattern is replaced with an underscore, the program is accepted.
|Tags||No tags attached.|
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.
|It's good to see that the trunk typechecker now accepts the example.|
|Re-open before changing resolution.|
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
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.
|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|
|2015-12-11 19:18||xleroy||Status||resolved => closed|
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