Re: [Caml-list] "Type constructor b would escape its scope"

[Gregory Malecha <gmalecha@gmail.com>]

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Thanks so much.

On Mon, Mar 28, 2016, 1:08 AM Gabriel Scherer <gabriel.scherer@gmail.com>
wrote:

> Locally abstract types scope over the definition body where they can
> be used as type names or to refine GADTs by pattern-matching.
>
> module type Ty = sig type t end
>
> (* we need the scoping of the locally abstract type in this example;
>    note that (type a) would work as well *)
> let mkTy : type a . unit -> (module Ty with type t = a) =
>   fun () ->
>     let module T = struct type t = a end in
>     (module T)
>
> > val mkTy : unit -> (module Ty with type t = 'a) = <fun>
>
> (* a type variable 'a only scopes over the type annotation *)
> let mkTy : 'a . unit -> (module Ty with type t = 'a) =
>   fun () ->
>     let module T = struct type t = 'a end in
>     (module T)
>
> >  let module T = struct type t = 'a end in
> >                                 ^^
> > Error: Unbound type parameter 'a
>
>
> type _ tag = Int : int tag | Prod : 'a tag * 'b tag -> ('a * 'b) tag
>
> (* we need the ability to be refined by GADT matching here *)
> let rec default : type a . a tag -> a =
>   function
>   | Int -> 0
>   | Prod (ta, tb) -> (default ta, default tb)
>
> > val default : 'a tag -> 'a = <fun>
>
> (* a type variable just unifies with the first case and fails on the
> second *)
> let rec default : 'a . 'a tag -> 'a =
>   function
>   | Int -> 0
>   | Prod (ta, tb) -> (default ta, default tb)
> >   ^^^^^^^^^^^^^
> > Error: This pattern matches values of type ($0 * $1) tag
> >        but a pattern was expected which matches values of type int tag
> >        Type $0 * $1 is not compatible with type int
>
> (* the difference by "(type a) ..." and "type a . ..." is that the latter
>    allows polymorphic recursion (just as "'a . ..."), so this example fails
>    with just (type a) *)
> let rec default (type a) : a tag -> a =
>   function
>   | Int -> 0
>   | Prod (ta, tb) -> (default ta, default tb)
> >                             ^^
> > Error: This expression has type $0 tag but an expression
> > was expected of type 'a
> > The type constructor $0 would escape its scope
>
>
> On Mon, Mar 28, 2016 at 6:20 AM, Gregory Malecha <gmalecha@gmail.com>
> wrote:
> > Thanks. I'm not sure I understand the difference between the two though.
> Is
> > there an example showing when they produce different results?
> >
> > On Sun, Mar 27, 2016 at 6:12 PM, Jacques Garrigue
> > <garrigue@math.nagoya-u.ac.jp> wrote:
> >>
> >> On 2016/03/28 03:52, Gregory Malecha wrote:
> >> >
> >> > Thanks, Leo --
> >> >
> >> > This is exactly what I needed to know. I thought that the (type a)
> >> > annotation was forcing the function to be polymorphic. For future
> reference,
> >> > is there any way to write the annotation (that forces the function to
> be
> >> > polymorphic) without having to write explicit ‘fun's?
> >>
> >> You should either write
> >>
> >>   let generic_search_stream : ‘b. int option -> (internal_result, ‘b)
> >> result_stream = …
> >>
> >> or
> >>
> >>   let generic_search_stream : type b. int option -> (internal_result, b)
> >> result_stream = …
> >>
> >> where the latter one both requires polymorphism and defines a locally
> >> abstract type,
> >> i.e. probably the behavior you expect with (type b).
> >>
> >> Jacques
> >>
> >
> >
> >
> > --
> > gregory malecha
>

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