Re: [Caml-list] Cannot safely evaluate the definition of the recursively-defined module

[Hugo Ferreira <hmf@inescporto.pt>]

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
>   let add e s = ASet.add e s
> 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
>   let add t q = An_A.add t q
> end
> 
> module Wrap_A1 = Make_A( A2 )
> ----
> 
> Cheers,
> -Mike
> 
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