* [Caml-list] Lack of an empty variant type
@ 2020-11-13 11:19 François Pottier
[not found] ` <9F66A427-C0B7-44B6-AE07-330D031FD631@gmail.com>
0 siblings, 1 reply; 2+ messages in thread
From: François Pottier @ 2020-11-13 11:19 UTC (permalink / raw)
To: OCaML Mailing List
Hello,
I was surprised today to see that this code is rejected:
# let f (x : [< `A]) = ();;
val f : [< `A ] -> unit = <fun>
# let g (x : [< `B]) = ();;
val g : [< `B ] -> unit = <fun>
# fun b x -> if b then f x else g x;;
Error: This expression has type [< `A ]
but an expression was expected of type [< `B ]
These two variant types have no intersection
The type-checker rejects this code, even though it is safe,
and its type should be [< ] -> unit, where [< ] is the empty
variant type.
I imagine that the type-checker wants to be my friend and
wants to prevent me from defining a useless function.
However, in the real use case I have in mind, I am abusing
polymorphic variants to encode sets at the type level, and
the lack of an empty set is painful!
Just a random remark,
--
François Pottier
francois.pottier@inria.fr
http://cambium.inria.fr/~fpottier/
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [Caml-list] Lack of an empty variant type
[not found] ` <9F66A427-C0B7-44B6-AE07-330D031FD631@gmail.com>
@ 2020-11-17 6:38 ` François Pottier
0 siblings, 0 replies; 2+ messages in thread
From: François Pottier @ 2020-11-17 6:38 UTC (permalink / raw)
Cc: Mailing List OCaml
Le 17/11/2020 à 02:02, Jacques Garrigue a écrit :
> Can’t you work around that by adding a dummy element to your sets?
Hi Jacques, yes, I can; but it adds visual clutter to types that
are already difficult to read. Not a big deal, I guess.
--
François Pottier
francois.pottier@inria.fr
http://cambium.inria.fr/~fpottier/
^ permalink raw reply [flat|nested] 2+ messages in thread
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2020-11-13 11:19 [Caml-list] Lack of an empty variant type François Pottier
[not found] ` <9F66A427-C0B7-44B6-AE07-330D031FD631@gmail.com>
2020-11-17 6:38 ` François Pottier
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