Every Js.Date is equal to every other Js.Date

[mlms13]

Using == in Reason (or = in OCaml), all Js.Date.t values are equal:

let a = Js.Date.fromFloat 1553816680458.0
let b = Js.Date.fromFloat 0.0
let _ = Js.log (a = b) (* true *)

I tried looking through the caml_equal function, but I got a bit lost. :)

[bobzhang]

hi, = is unsafe unless you know what you do. We have a warning flag (102) to warn unsafe usage of =, we accept patches to make the default behavior less surprising

[yawaramin]

More details on the unsafety here https://blog.janestreet.com/the-perils-of-polymorphic-compare/

[cristianoc]

For reference: this is the point where dates would be handled:
https://github.com/BuckleScript/bucklescript/blob/fafc9fd350a29275bb523c17564aed27879efc96/jscomp/runtime/caml_obj.ml#L328

After O.isArray one would check O.isDate which presumably would use instanceof Date.
Date equality does not work with == in JS so one would need to use <= and >=.

[bobzhang]

It makes sense to make the default less surprising, we are going to have this fixed in next release

[yawaramin]

One option to kind of cover this and other surprises with polymorphic compares is to by default enable 102 as a warning. Actually, now that I think about it, I want to propose some stricter warnings and errors by default anyway. I'll open a separate issue.

[bobzhang]

@yawaramin it used to be default, but people don't like it ..

[yawaramin]

@bobzhang that makes sense actually–I had it turned on for a bit and realized it's not great because the warning doesn't go away–it keeps appearing even if the file isn't touched afterwards 😅