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

[Enrico Tassi <enrico.tassi@inria.fr>]

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.

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,
-- 
Enrico Tassi

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