Re: [Caml-list] Error: In this definition, a type variable cannot be deduced from the type parameters.

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

2016/03/06 10:33, Goswin von Brederlow <goswin-v-b@web.de>:
> 
> Hi,
> 
> if I use the type aliases in the commented out lines then I get:
> 
>    Error: In this definition, a type variable cannot be deduced
>    from the type parameters.
> 
> To me the types sure do look alike and nothing is private or hidden my
> a signature. So what am I missing?
> 
> MfG
> 	Goswin

The problem is that you are using abstract types in separate modules.
As a result, the type system cannot recognize them as injective, and
gives the above error message.
You should either make it a datatype or a structural type:
  type +'a qObject' = Q of 'a
or
  type +'a qObject' = [`QObject' of 'a]

Jacques Garrigue

> -------------------------------------------------------------------------
> module Proxy = struct
>  type +'a t
> end
> 
> module QObject = struct
>  type +'a qObject'
>  type +'a t = 'a qObject' Proxy.t
> 
>  class ['a] qObject proxy = object
>    constraint 'a = 'b qObject' as 'a
>    method proxy : 'a Proxy.t = proxy
>  end
> end
> 
> module QWidget = struct
>  type +'a qWidget'
>  type +'a t = 'a qWidget' QObject.t
> 
>  external add : 'a qWidget' QObject.qObject' Proxy.t -> 'b qWidget' QObject.qObject' Proxy.t -> unit = "add"
>  (*
>    external add : 'a t -> 'b qWidget' QObject.t -> unit = "add"
>  *)
>  class ['a] qWidget proxy = object
>    constraint 'a = 'b qWidget' QObject.qObject' as 'a
>    inherit ['a] QObject.qObject proxy
>    method add : 'c 'd . ('d qWidget' QObject.qObject' #QObject.qObject as 'c) -> unit = fun w -> add proxy w#proxy
>  end
> end
> 
> module QButton = struct
>  type +'a qButton'
>  type +'a t = 'a qButton' QWidget.t
> 
>  external set : 'a qButton' QWidget.qWidget' QObject.qObject' Proxy.t -> 'b QWidget.qWidget' QObject.qObject' Proxy.t -> unit = "set"
>  (*
>    external set : 'a t -> 'b QWidget.t -> unit = "set"
>  *)
>  class ['a] qButton proxy = object
>    constraint 'a = 'b qButton' QWidget.qWidget' QObject.qObject' as 'a
>    inherit ['a] QWidget.qWidget proxy
>    method set : 'c 'd . ('c QWidget.qWidget' QObject.qObject' #QWidget.qWidget as 'd) -> unit = fun w -> set proxy w#proxy
>    method text = "button"
>  end
> end
> 
> let qObject_proxy : unit QObject.t = Obj.magic 0
> let qWidget_proxy : unit QWidget.t = Obj.magic 0
> let qButton_proxy : unit QButton.t = Obj.magic 0
> let qObject = new QObject.qObject qObject_proxy
> let qWidget = new QWidget.qWidget qWidget_proxy
> let qButton = new QButton.qButton qButton_proxy
> let () = qWidget#add qWidget
> let () = qWidget#add qButton
> let () = qButton#set qWidget
> let () = qButton#set qButton
> let () = qButton#add qWidget
> let () = qButton#add qButton
> 
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