On Mon, 5 Jun 2000, Pierre Weis wrote:
> However, we get a strange difference in typing since, as you
> mentioned, the explicit ``as clause'' does not set up a typing
> connection between input and output :
>
> let (ensure_nil : 'a pliste -> 'b pliste) = function
> | `Cons (x, l) -> failwith "Not nil"
> | `Nil as l -> l;;
> val ensure_nil : 'a pliste -> 'b pliste = <fun>
>
> Jacques may explain us if the above suggested generalization scheme is
> used for identifiers bound in as clauses of patterns (and if not,
> which scheme is used ?)...
This discussion seems related to a recent typing difficulty I have had
with polymorphic variants.
I want to change the data associated to a particular constructor of a
variant and leave the others unmodified:
# let f = function `A -> `A () | _ as t -> t;;
Characters 41-42:
This expression has type [< `A | ..] but is here used with type
[> `A of unit]
Notice that the type for '_' includes `A when it obviously cannot. One
could include every possible expected type in place of '_' but that
removes one of the advantages of polymorphic variants: that the type can
be refined and extended easily.
While further exploring this issue I managed to get an uncaught exception
from the type checker:
# let f = function `A true -> () | `A 5 -> ();;
Uncaught exception: File "typing/parmmatch.ml", ...
Taking the conjuction of 'unit' and 'int' does seem to work however,
# let f = function `A -> () | `A 5 -> ();;
val f : [< `A of & int] -> unit = <fun>
but it prints the type without mentioning 'unit'.
Patrick Doane
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