SebCheersIs it intended?With OCaml version 4.00.1+dev2_2012-08-06 (4.00.1+short-types in Opam):I don't know if this directly related, or if it is actually intended, but this looks like a regression to me:Hi Jacques
# let f = function 0 -> `zero | 1 -> `one | _ -> `some;;
val f : int -> [> `one | `some | `zero ] = <fun>
# let g x = match f x with `one -> 1 | `zero -> 0;;
Error: This pattern matches values of type [< `one | `zero ][> `one | `some | `zero ]
but a pattern was expected which matches values of type
The first variant type does not allow tag(s) `somewhich is the nice behavior we were used to,but with OCaml version 4.01.0+dev10-2012-10-16 (a.k.a. 4.01.0dev+short-paths), the error has been downgraded to a warning:
# let f = function 0 -> `zero | 1 -> `one | _ -> `some;;
val f : int -> [> `one | `some | `zero ] = <fun>
# let g x = match f x with `one -> 1 | `zero -> 0;;
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
`some
val g : int -> int = <fun>
On Tue, Jan 15, 2013 at 12:59 AM, Jacques Garrigue <garrigue@math.nagoya-u.ac.jp> wrote:
Dear Camlers,
It is a bit unusual, but this message is about changes in trunk.
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.
Jacques Garrigue
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