[Caml-list] unnamed records in match on multiple constructors

[Caml-list] unnamed records in match on multiple constructors

[Gabriel Mayr]

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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 = {i:int};; #type t = A of a | B of a;; # let x = A {i=10};; # 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 = A of {i:int} | B of {i:int};; # let x = A {i=10};; # 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

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Re: [Caml-list] unnamed records in match on multiple constructors

[Gabriel Scherer]

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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 = A of {i:int} | B of {i:int};;

is morally equivalent to

  type t_internal_A = {i : int};;
  type t_internal_B = {i : int};;
  type t = 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 <gabriel.mayr@mail.de> 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 = {i:int};;
> #type t = A of a | B of a;;
> # let x = A {i=10};;
> # 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 = A of {i:int} | B of {i:int};;
> # let x = A {i=10};;
> # 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
>
> ------------------------------
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