Re: [Caml-list] private rows question

[Keiko Nakata <keiko@kurims.kyoto-u.ac.jp>]

From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>

> From: Keiko Nakata <keiko@kurims.kyoto-u.ac.jp>
> 
> > Why the following functor G is not typable?
> > I thought that g could have a type something like int -> [< `A | `B ].
> > 
> > module G(X:sig type t = private [< `A ] val f : int -> t end) = struct
> >   let g x = if x = 0 then `B else X.f x 
> > end
> 
> Since g may return `B, and probably `A (the probably concerns your
> intention), the expected type would be
>   g : int -> [> `A | `B ]
> Next, if you want f to have a private row type, then you should
> disconnect its type from the type of g (otherwise the type of g must
> be the same as f.) So I would expect something like:
>    let g x = if x = 0 then `B else (X.f x : [< `A] :> [> `A])

Thank you for the correction.
Obviously, the type checker is much more brilliant than me :-)
Private rows seem to give me more opportunities to reuse my program codes
than I expected.

Keiko NAKATA