Julia: Cannot use a Dates.Hour(1) and Dates.Month(1) as keys in the same Dict

[bluesmoon]

I get a method error if I try to create a Dict with Dates.Hour(1) and Dates.Month(1) as keys. All other combinations of keys work, but these two exact values cannot exist as keys together in the same Dict.

I understand why Dates.Hour(1) == Dates.Month(1) would throw a MethodError, but I do not understand why this should happen when adding them as keys to a Dict, and why the same error does not show up if, for example, we were to use Dates.Hour(2) or Dates.Year(1) instead.

More details at https://discourse.julialang.org/t/cannot-use-a-dates-hour-and-dates-month-as-keys-in-the-same-dict/

julia> versioninfo()
Julia Version 0.6.3-pre.2
Commit c011bb7 (2018-05-08 17:03 UTC)
DEBUG build
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)

julia> DATEPART_DISPLAY = Dict{Dates.Period, AbstractString}()
Dict{Base.Dates.Period,AbstractString} with 0 entries

julia> DATEPART_DISPLAY[Dates.Month(1)] = "%b %Y"
"%b %Y"

julia> DATEPART_DISPLAY[Dates.Hour(1)] = "%b %d %Y, %H:%M"
ERROR: MethodError: no method matching ==(::Base.Dates.Hour, ::Base.Dates.Month)
Closest candidates are:
  ==(::Union{Base.Dates.Day, Base.Dates.Hour, Base.Dates.Microsecond, Base.Dates.Millisecond, Base.Dates.Minute, Base.Dates.Nanosecond, Base.Dates.Second, Base.Dates.Week}, ::Union{Base.Dates.Month, Base.Dates.Year}) at dates/periods.jl:443
  ==(::Base.Dates.Period, ::Base.Dates.Period) at dates/periods.jl:63
  ==(::Any, ::Any) at operators.jl:52
  ...
Stacktrace:
 [1] ==(::Base.Dates.Hour, ::Base.Dates.Month) at ./dates/periods.jl:443
 [2] ht_keyindex2(::Dict{Base.Dates.Period,AbstractString}, ::Base.Dates.Hour) at ./dict.jl:366
 [3] setindex!(::Dict{Base.Dates.Period,AbstractString}, ::String, ::Base.Dates.Hour) at ./dict.jl:421

[simonbyrne]

As noted on Discourse, this actually works on 0.7. Not sure what changed.

[JeffBezanson]

There is still an error for Hour(1) == Month(1), due to a definition that explicitly throws a MethodError:

(==)(x::FixedPeriod, y::OtherPeriod) = throw(MethodError(==, (x, y)))
(==)(x::OtherPeriod, y::FixedPeriod) = throw(MethodError(==, (x, y)))

The usual style in julia (as assumed e.g. by Dict) is just to return false; I think these methods should be changed.

[nalimilan]

FWIW, we could also define isequal methods that return false, and keep throwing an error for ==, since dicts only need the former.

[StefanKarpinski]

We don't generally have == methods that throw an error because of the types of their arguments being incompatible. That is a design that we've considered in the past and that I've even advocated for from time to time, but as we have not adopted that approach, this case is just inconsistent.

[bluesmoon]

FWIW, I've currently worked around it in my code by defining the aforementioned isequal methods in my own code just before my Dict with a comment to remove it once this bug is fixed.

Thanks for the suggestion, and I hope it does get fixed at some point.

[ninjin]

Patched it in one of my branches, the original discussion that lead to the introduction of the “offending” lines were in #21378 by @omus. Happy to receive comments or order to submit a PR, especially from someone with opinions on the hellishly complicated topic of time – personally I am just going through issues, finding this to be a silly gotcha, and coding late on a Friday night for kicks.

[JeffBezanson]

Yes a PR with that would be fine. Even better if it can be fixed by just deleting those methods.

[ninjin]

Tried deleting, but it tries to promote and goes down in flames. Filing PR to move implementation specific discussion there.

[JeffBezanson]

Fixed by #27165.

[bluesmoon]

Is this going to be backported to 0.6?

[StefanKarpinski]

It seems like a behavioral change rather than a bug fix, no?