Constraints in module types

Constraints in module types

[Alessandro Baretta]

# type foo = [ `Foo of foo ];;
type foo = [ `Foo of foo ]
# module type BAR = sig type bar constraint bar = [ > foo ] end;;
The type constraints are not consistent.
Type bar is not compatible with type [> foo ]

Is there a good reason for prohibiting the above declarations? Could I achieve a 
similar effect with private row types?

Alex

Re: [Caml-list] Constraints in module types

[Jacques Garrigue]

From: Alessandro Baretta <a.baretta@barettadeit.com>

> # type foo = [ `Foo of foo ];;
> type foo = [ `Foo of foo ]
> # module type BAR = sig type bar constraint bar = [ > foo ] end;;
> The type constraints are not consistent.
> Type bar is not compatible with type [> foo ]
> 
> Is there a good reason for prohibiting the above declarations? Could I achieve a 
> similar effect with private row types?

Not a "similar" effect, as the meaning of the above code is: define an
abstract type bar, and check that this abstract type is unifiable with
[> foo] (by definition impossible, so this might just be a syntax
error...)

On the other hand

  module type BAR = sig type bar = private [> foo] end

means "require the type bar to be an instance of type [> foo]".
I suppose this is what you intended?

Jacques

Re: [Caml-list] Constraints in module types

[Alessandro Baretta]

Jacques Garrigue wrote:
> From: Alessandro Baretta <a.baretta@barettadeit.com>

> On the other hand
> 
>   module type BAR = sig type bar = private [> foo] end
> 
> means "require the type bar to be an instance of type [> foo]".
> I suppose this is what you intended?
> 
> Jacques

What a man! Thank you! This is exactly what I was trying to express.

Alex


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