Re: [Caml-list] How to rename a record field

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2018/09/06 22:03, Enrico Tassi wrote:
> 
> On Thu, Sep 06, 2018 at 01:48:23PM +0200, Gabriel Scherer wrote:
>>  MPR#7102: Ability to re-export a variant definition with renamed
>> constructors?
>>  https://caml.inria.fr/mantis/view.php?id=7102
> 
> Thanks for the pointer, but I'm not sure I understand how the arguments
> made there apply to records.

Records being the dual of variants, all the arguments apply.
Namely, if the name is allowed to change, field access can no longer
be seen as the intuitive projection (by name rather than position, since
only name information is available in the source code).
Field name disambiguation works in the same way too.

There is also the same problem with GADTs assuming that two
record types with different field names are distictinct.

The fact the representation is identical is just an implementation detail.
Tuples too share the same representation.

Jacques Garrigue

> My intuition is that, given that records are compared by name, there
> should be no difference between adding the equation t = old or
> casting (with Obj.magic) each and every term of type old to type t.
> To my knowledge the two records have the very same representation at run
> time, that does not depend on the names of the fields but just their
> types.
> 
> I've even tried to disable the check in typing/includecore.ml around
> line 230 and the following silly test seems to work.
> 
>  type old = { bad : int; stuff : bool }
>  type t = old = { good : int; stuff : bool }
> 
>  let rb = { bad = 3; stuff = false }
>  let rg = { good = 3; stuff = false }
> 
>  let _ = Printf.printf "%b\n" rb.stuff
>  let _ = Printf.printf "%b\n" rg.stuff
>  let _ = Printf.printf "%d\n" rb.bad
>  let _ = Printf.printf "%d\n" rg.good
>  let _ = Printf.printf "%d\n" rg.bad
>  let _ = Printf.printf "%d\n" rb.good
>  let _ = Printf.printf "%d\n" { rb with good = rb.bad + 1 }.bad
>  let _ = Printf.printf "%b\n" { rb with stuff = not rg.stuff }.stuff
> 
> Where is my intuition wrong?
> 
> Why records needs to be compared by name and not by (type) structure?
> Sorry if the question is silly, but I really don't know.
> 
> Best,




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