编译此程序时:
module type Inc =
sig
type t
val inc : t -> t
end
module type Dec =
sig
type t
val dec : t -> t
end
module Merger (I : Inc) (D : Dec with type t = I.t) =
struct
let merge x = x |> I.inc |> D.dec
end
module IntInc : Inc =
struct
type t = int
let inc x = x + 10
end
module IntDec : Dec =
struct
type t = int
let dec x = x - 8
end
module Combiner = Merger (IntInc) (IntDec)
I get the following error:
File "simple.ml", line 30, characters 35-41:
Error: Signature mismatch:
Modules do not match:
sig type t = IntDec.t val dec : t -> t end
is not included in
sig type t = IntInc.t val dec : t -> t end
Type declarations do not match:
type t = IntDec.t
is not included in
type t = IntInc.t
File "simple.ml", line 13, characters 38-50: Expected declaration
File "simple.ml", line 9, characters 13-14: Actual declaration
我认为D: Dec with type t = I.t
的约束条件会确保D.t = I.t
。为什么这不是事实呢?
更有趣的是,当我删除module Combiner = Merger (IntInc) (IntDec)
这一行时,它可以编译通过而没有错误。
我的问题是:我做错了什么?