Value comparison does not work for Date.Date

[lukewestby]

Simplest reproducible example, doable in the REPL:

import Date
(Date.fromTime 1) == (Date.fromTime 2) -- => True

This is happening because instances of JS Date are "exotic" objects and not primitives. They have no own properties and end up falling through both the strict equality and object cases for Utils.eq(). They're a rather strange edge case as far as JS object comparison goes, as they need to be coerced to their Number representation first with + or valueOf() first.

The workaround for now that I've been using is to do the same sort of coercion, just in Elm:

import Date
date1 = Date.fromTime 1
date2 = Date.fromTime 2
(Date.toTime date1) == (Date.toTime date2) -- => False

But IMVHO it seems like if Date is going to be a built-in data type for the language, it should be possible to compare instances reliably without transformation.

The way I would plan to handle this is by adding a top-level case to eq to check x instanceof Date && y instanceof Date and then compare x.valueOf() === y.valueOf(). If that plan makes sense to you I can submit a PR 😄

Thanks!

[athanclark]

I'm not sure if this is related, but I've needed to compare Dates and have been disappointed that they're not natively comparable. I had to build this verbose utility function for comparing months, may you scrape it for success!

compareMonth : Month -> Month -> Order
compareMonth m1 m2 =
  case (m1, m2) of
    (Jan, Jan) -> EQ
    (Jan, _)   -> LT
    (Feb, Jan) -> GT
    (Feb, Feb) -> EQ
    (Feb, _)   -> LT
    (Mar, Jan) -> GT
    (Mar, Feb) -> GT
    (Mar, Mar) -> EQ
    (Mar, _)   -> LT
    (Apr, Jan) -> GT
    (Apr, Feb) -> GT
    (Apr, Mar) -> GT
    (Apr, Apr) -> EQ
    (Apr, _)   -> LT
    (May, Jan) -> GT
    (May, Feb) -> GT
    (May, Mar) -> GT
    (May, Apr) -> GT
    (May, May) -> EQ
    (May, _)   -> LT
    (Jun, Jan) -> GT
    (Jun, Feb) -> GT
    (Jun, Mar) -> GT
    (Jun, Apr) -> GT
    (Jun, May) -> GT
    (Jun, Jun) -> EQ
    (Jun, _)   -> EQ
    (Jul, Dec) -> LT
    (Jul, Nov) -> LT
    (Jul, Oct) -> LT
    (Jul, Sep) -> LT
    (Jul, Aug) -> LT
    (Jul, Jul) -> EQ
    (Jul, _)   -> GT
    (Aug, Dec) -> LT
    (Aug, Nov) -> LT
    (Aug, Oct) -> LT
    (Aug, Sep) -> LT
    (Aug, Aug) -> EQ
    (Aug, _)   -> GT
    (Sep, Dec) -> LT
    (Sep, Nov) -> LT
    (Sep, Oct) -> LT
    (Sep, Sep) -> EQ
    (Sep, _)   -> GT
    (Oct, Dec) -> LT
    (Oct, Nov) -> LT
    (Oct, Oct) -> EQ
    (Oct, _)   -> GT
    (Nov, Dec) -> LT
    (Nov, Nov) -> EQ
    (Nov, _)   -> GT
    (Dec, Dec) -> EQ
    (Dec, _)   -> GT

[athanclark]

I'm not sure if this is related, but I've needed to compare Dates and have been disappointed that they're not natively comparable. I had to build this verbose utility function for comparing months, may you scrape it for success!

compareMonth : Month -> Month -> Order
compareMonth m1 m2 =
  case (m1, m2) of
    (Jan, Jan) -> EQ
    (Jan, _)   -> LT
    (Feb, Jan) -> GT
    (Feb, Feb) -> EQ
    (Feb, _)   -> LT
    (Mar, Jan) -> GT
    (Mar, Feb) -> GT
    (Mar, Mar) -> EQ
    (Mar, _)   -> LT
    (Apr, Jan) -> GT
    (Apr, Feb) -> GT
    (Apr, Mar) -> GT
    (Apr, Apr) -> EQ
    (Apr, _)   -> LT
    (May, Jan) -> GT
    (May, Feb) -> GT
    (May, Mar) -> GT
    (May, Apr) -> GT
    (May, May) -> EQ
    (May, _)   -> LT
    (Jun, Jan) -> GT
    (Jun, Feb) -> GT
    (Jun, Mar) -> GT
    (Jun, Apr) -> GT
    (Jun, May) -> GT
    (Jun, Jun) -> EQ
    (Jun, _)   -> EQ
    (Jul, Dec) -> LT
    (Jul, Nov) -> LT
    (Jul, Oct) -> LT
    (Jul, Sep) -> LT
    (Jul, Aug) -> LT
    (Jul, Jul) -> EQ
    (Jul, _)   -> GT
    (Aug, Dec) -> LT
    (Aug, Nov) -> LT
    (Aug, Oct) -> LT
    (Aug, Sep) -> LT
    (Aug, Aug) -> EQ
    (Aug, _)   -> GT
    (Sep, Dec) -> LT
    (Sep, Nov) -> LT
    (Sep, Oct) -> LT
    (Sep, Sep) -> EQ
    (Sep, _)   -> GT
    (Oct, Dec) -> LT
    (Oct, Nov) -> LT
    (Oct, Oct) -> EQ
    (Oct, _)   -> GT
    (Nov, Dec) -> LT
    (Nov, Nov) -> EQ
    (Nov, _)   -> GT
    (Dec, Dec) -> EQ
    (Dec, _)   -> GT