[Caml-list] Sample code doesn't work

[Caml-list] Sample code doesn't work

[Rolf Wester]

Hi,

I have a problem with the following simple example:

let rec make_account wert0 = 
  let wert = wert0 in
  let add wert_ =
    make_account (wert+.wert_)
  in   
  let sub wert_ =
    make_account (wert-.wert_)
  in 
  let get () = 
    wert 
  in
  (add, sub, get);;

When trying to evaluate this I get the error message:

This expression has type
  (float -> 'a * 'b * 'c) * (float -> 'a * 'b * 'c) * (unit -> float)
but is here used with type 'a * 'b * 'c

How can I make this work?


Rolf Wester
-------------------------------------
Rolf Wester
wester@ilt.fhg.de
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Re: [Caml-list] Sample code doesn't work

[Alain Frisch]

On Mon, 16 Jul 2001, Rolf Wester wrote:

> let rec make_account wert0 = 
>   let wert = wert0 in
>   let add wert_ =
>     make_account (wert+.wert_)
>   in   
>   let sub wert_ =
>     make_account (wert-.wert_)
>   in 
>   let get () = 
>     wert 
>   in
>   (add, sub, get);;
> 
> When trying to evaluate this I get the error message:
> 
> This expression has type
>   (float -> 'a * 'b * 'c) * (float -> 'a * 'b * 'c) * (unit -> float)
> but is here used with type 'a * 'b * 'c
> 
> How can I make this work?

You need recursive types to be able to unify these two type
expressions. It is often the case when you want, as in this
example, simulate the object paradigm.

Just add the -rectypes switch to the toplevel/compiler command line.


-- 
  Alain Frisch

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