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bh=0KN9h0bcymHb5XIxvTEb7x21ybjhTKCysrGtQ9hiAhs=; b=rqiOdsLrx86Ys9LyzQiBa6sluaVLalUWebb5NWvbS1FzkGck1lxmUQqvswpLvoAx/f F/6udjGChFSAIZCyDl12v1m3kyxd5Fv8zP8oYOnSoDJnDdrY76TSiV4ipWY8JgKtLznZ wqaU3FZmezXXAjm8KQpQzajFx1bUWB0cY3fZBplHa/uGQ4co/YCKO9RBgPaLGKemYL1A N3AfC5OmDxNpTXzWENS59xwPdPyF5jDFcwgt4skH+r5hMVFWvr68ooXlFgOKkX7RnLMD +WQt+Zpri+mGeixfLU5fvJMTZv88ZFAZBtLXBwMJi6RYzDCcKgQVvWtAv04Z2pC83S6u X+VQ== X-Gm-Message-State: AKaTC02RNyElCtcyg+X0k8S+yKy8HaMtfpC0DEY+/kqt/8n4dw5BlVm3DmecHxf9Vu4rg0E7TgTZhjbJM2WnmA== X-Received: by 10.36.217.79 with SMTP id p76mr1837924itg.16.1481814554840; Thu, 15 Dec 2016 07:09:14 -0800 (PST) MIME-Version: 1.0 Received: by 10.79.140.146 with HTTP; Thu, 15 Dec 2016 07:08:34 -0800 (PST) In-Reply-To: <20161215141240.01BE740092@smtp03.mail.de> References: <20161215141240.01BE740092@smtp03.mail.de> From: Gabriel Scherer Date: Thu, 15 Dec 2016 10:08:34 -0500 Message-ID: To: Gabriel Mayr Cc: caml users Content-Type: multipart/alternative; boundary=001a1145ed9e2a12240543b3d67b Subject: Re: [Caml-list] unnamed records in match on multiple constructors --001a1145ed9e2a12240543b3d67b Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable The issue is "what is the type of an inline record"? Internally each inline record in a constructor declaration has its own type, so in particular two different records have distinct types. One way to think of it is that type t =3D A of {i:int} | B of {i:int};; is morally equivalent to type t_internal_A =3D {i : int};; type t_internal_B =3D {i : int};; type t =3D A of t_internal_A | B of t_internal_B;; And then (function (A e | B e) -> e.i) does not type-check either -- with the same error message. This comes from the fact that, in OCaml (unlike in SML), records are nominal, have an identity: declaring another record type with the same fields creates a new, incompatible type. So the answer is: this is a natural, expected consequence of how inline records and how records work in OCaml. On Thu, Dec 15, 2016 at 9:12 AM, Gabriel Mayr wrote: > > Hi, > > I'm using utop version 1.19.3 (using OCaml version 4.03.0). > > Using an *named* record in a type definition and afterwards match on > multiple constructors *works:* > # type a =3D {i:int};; > #type t =3D A of a | B of a;; > # let x =3D A {i=3D10};; > # match x with > | A e | B e -> print_int e.i;; > > > Using an *unnamed* record in a type definition and afterwards match on > multiple constructors *does not work:* > # type t =3D A of {i:int} | B of {i:int};; > # let x =3D A {i=3D10};; > # match x with > | A e | B e -> print_int e.i;; > > "Error: The variable e on the left-hand side of this or-pattern has type > t.A but on the right-hand side it has type t.B" > > > > I don't know if this is behaviour of OCaml is desired or it's a fault. > > Hope I could help you with this little hint. > > Best regards, > > Gabriel > > ------------------------------ > FreeMail powered by mail.de - *mehr Sicherheit, Seriosit=C3=A4t und Komfo= rt* --001a1145ed9e2a12240543b3d67b Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
The issue is "what is the type of an inline record&qu= ot;? Internally each inline record in a constructor declaration has its own= type, so in particular two different records have distinct types. One way = to think of it is that

=C2=A0 type t =3D A of {i:int} | B of {i:int};;

is m= orally equivalent to

=C2=A0 type t_internal_A =3D {i : in= t};;
=C2=A0 type t_internal_B =3D {i : int};;
= =C2=A0 type t =3D A of t_internal_A | B of t_internal_B;;

And then (function (A e | B e) -> e.i) does not type-check either -- wi= th the same error message. This comes from the fact that, in OCaml (unlike = in SML), records are nominal, have an identity: declaring another record ty= pe with the same fields creates a new, incompatible type.

So the answer is: this is a natural, expected consequence of how inline re= cords and how records work in OCaml.

On Thu, Dec 15, 2016 at 9:12 AM, Gabriel Mayr <gabriel.mayr@mail.de> wrote:
=C2=A0
Hi,
=C2=A0
I'm using utop version 1.19.3 (using OCaml version 4.03.0).
=C2=A0
Using an named record in a type definition and afterwards match= on multiple constructors works:
# type a =3D {i:int};;
#type t =3D A of a | B of a;;
# let x =3D A {i=3D10};;
# match x with
| A e | B e -> print_int e.i;;
=C2=A0
=C2=A0
Using an unnamed record in a type definition and afterwards mat= ch on multiple constructors does not work:
# type t =3D A of {i:int} | B of {i:int};;
# let x =3D A {i=3D10};;
# match x with
| A e | B e -> print_int e.i;;

"Error: The variable e on the left-hand side of this or-patte= rn has type t.A but on the right-hand side it has type t.B"
=C2=A0
=C2=A0
=C2=A0
I don't know if this is behaviour of OCaml is desired or it's = a fault.
=C2=A0
Hope I could help you with this little hint.
=C2=A0
Best regards,
=C2=A0
Gabriel

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