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Cannot safely evaluate the definition of the recursively-defined module
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 Date: -- (:) From: Hugo Ferreira Subject: Re: [Caml-list] Cannot safely evaluate the definition of the recursively-defined module
Michael Furr wrote:
>
> On Wed, 18 Mar 2009, Hugo Ferreira wrote:
>
>> I hope this is not a beginners questions.
>> I am trying to reuse code via functors,
>> however I am getting the following error:
>>
>> Cannot safely evaluate the definition of the recursively-defined module
>> (refers to "AA.empty" when implemented as a constant value)
>>
>> I circumvented the problem by not using a
>> constant value but a function instead. As I
>> understand it this may cause run-time errors.
>> My question is: is their any way to make the
>> following example work.
>
> If you only need to store a constant value in the module, then you can
> get around the restriction by splitting the module into a
> (recursively-)safe module, and an extension that adds the constants.
> Here is a slightly modified version of your code showing the
> transformation:
>

Ok, I understand this.

Thanks,
Hugo F.

> ----
> module type AA_Safe =
> sig
>   type q
>   type t = string
>
>   val compare: t -> t -> int
>   val add: t -> q -> q
>   (* omit empty here, since it is not "safe" *)
> end
>
> module rec A1 : AA_Safe with type q = ASet.t  =
> struct
>   type q = ASet.t
>   type t = string
>   let compare s1 s2 = Pervasives.compare s1 s2
> end
> and ASet : Set.S with type elt = A1.t = Set.Make(A1)
>
> (* now create the full module *)
> module type AA = sig
>   include AA_Safe
>   val empty: q
> end
>
> module A2 : AA = struct
>   include A1
>   let empty = ASet.empty
> end
>
> module type Wrap_A =
> sig
>   type t
>   type q
>
>   val init: q
>   val add: t -> q -> q
> end
> module Make_A (An_A : AA) : Wrap_A
>   =
> struct
>   type t = An_A.t
>   type q = An_A.q
>
>   (*let init = ASet.empty*)
>   let init = An_A.empty
> end
>
> module Wrap_A1 = Make_A( A2 )
> ----
>
> Cheers,
> -Mike
>
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