Re: [Caml-list] Anonymous sum types in functors

[David Allsopp <dra-news@metastack.com>]

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On 23 Jun 2013, at 10:13, "Damien Guichard" <alphablock@orange.fr<mailto:alphablock@orange.fr>> wrote:




> a) Is there a way to do it where you can end up with type elt = A | B (I think the answer is no?)

> b) Is there a syntactically lighter way to write the module definition?



My own quick & dirty hack :

module FlagSet = Set.Make(struct type t = [`A | `B] let compare = compare end)

Sadly not - the values are in use with C bindings and polymorphic variants would rather defeat the point of what I'm doing!

What would be nice would be an equivalent 'immediate' syntax for variant types - but its only use would be this context, I expect!


David


- damien


I couldn't think of a better way to describe what I'm trying to do, so forgive the possibly strange subject!



In:



module IntSet = Set.Make(struct type t = int let compare = compare end)



the resulting signature is:



sig

  type elt = int

  type t

  ...



but in:



module FlagSet = Set.Make(struct type t = A | B let compare = compare end)



the resulting signature is:



sig

  type elt

  type t

  ...



i.e. the constructors are hidden (I can see why, but presumably it is a special case in the type checker?) and the module is essentially useless. I don't want to define the type external to the module - the idea is that I'd be able to write Flag1Set.add Flag1Set.CommonFlag Flag1Set.empty and Flag2Set.add Flag2Set.CommonFlag Flag2Set.empty, etc.



I can work around this by writing:



module FlagSet =

  struct

    type flag = A | B

    include Set.Make(struct type t = flag let compare = compare end)

  end



where the resulting signature is:



sig

  type flag = A | B

  type elt = flag

  type t

  ...



but I'm wondering:



a) Is there a way to do it where you can end up with type elt = A | B (I think the answer is no?)

b) Is there a syntactically lighter way to write the module definition?





David


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