From: Ben Millwood <bmillwood@janestreet.com>
To: caml users <caml-list@inria.fr>
Subject: [Caml-list] Can we have more flexible destructive type substitution?
Date: Fri, 24 Jul 2015 15:25:31 +0100 [thread overview]
Message-ID: <CA+MHO51FYs9xwQ-XJ6O86sUL7Xi3A5xKM8QEhN3a7Et6bYGJ4g@mail.gmail.com> (raw)
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Currently, for [sig ... end with type lhs := rhs] to be valid, rhs needs to
be an application of a type constructor to the same type parameters as lhs.
I'd like to do, at a minimum, [sig ... end with type 'a t := 'a], but this
is currently forbidden. Are there conceptual obstacles to permitting
arbitrary type expressions on the RHS, or are they simply not implemented
yet?
Note that I can do [type 'a id = 'a] and then [... with type 'a t := 'a
id], but it's clumsy to have to define an auxiliary type for this and it's
not always easy to find an appropriate place to do so.
In particular, I have modules with a blocking interface and an Async
interface, so I define:
module type Thing = sig
type 'a result
val read_thing : in_channel -> thing result
val write_thing : out_channel -> unit result
end
thing_async.mli:
include Thing with type 'a result := 'a Deferred.t
thing_or_error.mli:
include Thing with type 'a result := 'a Or_error.t
thing_exn.mli:
include Thing with type 'a result := 'a (* fails :( *)
Having Thing_exn.id be a useless type constructor would be pretty sad.
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next reply other threads:[~2015-07-24 14:25 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-24 14:25 Ben Millwood [this message]
2015-07-24 14:37 ` Jacques Garrigue
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