[Caml-list] Anonymous sum types in functors

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

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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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