Hi,
I'm writing a module that reimplements the option type without
redirection. For that I'm mapping None to a value with all bits set to
0, the NULL pointer in C. For that I have a little helper function:
external make_null : unit -> 'a t = "caml_shallow_null"
But now I want to also have a polymorphic value for NULL:
module Make : sig
type 'a t
val make_list : unit -> 'a list
val make_null : unit -> 'a t
end = struct
type 'a t
let make_list () = []
external make_null : unit -> 'a t = "caml_shallow_null"
end
module Shallow : sig
type 'a t
val list : 'a list
val null : 'a t
end = struct
type 'a t = 'a Make.t
let list = Make.make_list ()
let null = Make.make_null ()
end
File "shallow.ml", line 15, characters 6-102:
Error: Signature mismatch:
Modules do not match:
sig
type 'a t = 'a Make.t
val list : 'a list
val null : '_a Make.t
end
is not included in
sig type 'a t val list : 'a list val null : 'a t end
Values do not match:
val null : '_a Make.t
is not included in
val null : 'a t
What makes the Make.make_list differ from Make.make_null?
Is there a way that I can specify that null is still polymorphic or does
that only work for constructors like None and compiler magic like []?
----------------------------------------------------------------------
And here something odd:
module Make : sig
type 'a t = 'a list
val make_list : unit -> 'a list
val make_null : unit -> 'a t
end = struct
type 'a t = 'a list
let make_list () = []
external make_null : unit -> 'a t = "caml_shallow_null"
end
module Shallow : sig
type 'a t
val list : 'a list
val null : 'a t
end = struct
type 'a t = 'a Make.t
let list = Make.make_list ()
let null = Make.make_null ()
end
compiles fine. But
module Make : sig
type 'a t = private 'a list
val make_list : unit -> 'a list
val make_null : unit -> 'a t
end = struct
type 'a t = 'a list
let make_list () = []
external make_null : unit -> 'a t = "caml_shallow_null"
end
module Shallow : sig
type 'a t
val list : 'a list
val null : 'a t
end = struct
type 'a t = 'a Make.t
let list = Make.make_list ()
let null = Make.make_null ()
end
fails again. Just making the Make.t private makes it fail.
MfG
Goswin
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