Improve error handling for DateTime.to_iso8601

[capablemonkey]

Environment

• Elixir & Erlang versions (elixir --version): Elixir 1.5.0 & Erlang/OTP 20 [erts-9.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

• Operating system: Mac OS X 10.11.6

Current behavior

Without realizing it, I passed in a NaiveDateTime into DateTime.to_iso8601(), and got back a cryptic error message.

iex(12)> DateTime.to_iso8601(NaiveDateTime.utc_now())
** (ArgumentError) DateTime.to_iso8601/2 expects format to be :extended or :basic, got: :extended
    (elixir) lib/calendar/datetime.ex:345: DateTime.to_iso8601/2

This happens because the fallback pattern assumes that if no other function patterns were matched, there must be a fault with the second argument. However, in this case the second argument got the default value and the issue was with the first argument's type.

elixir/lib/elixir/lib/calendar/datetime.ex

Lines 327 to 346 in 8914d2c

	def to_iso8601(datetime, format \\ :extended)
	def to_iso8601(%{calendar: Calendar.ISO, year: year, month: month, day: day,
	hour: hour, minute: minute, second: second, microsecond: microsecond,
	time_zone: time_zone, zone_abbr: zone_abbr, utc_offset: utc_offset, std_offset: std_offset}, format) when format in [:extended, :basic] do
	Calendar.ISO.datetime_to_iso8601(year, month, day, hour, minute, second, microsecond,
	time_zone, zone_abbr, utc_offset, std_offset, format)
	end
	def to_iso8601(%{calendar: _, year: _, month: _, day: _,
	hour: _, minute: _, second: _, microsecond: _,
	time_zone: _, zone_abbr: _, utc_offset: _, std_offset: _} = datetime, format) when format in [:extended, :basic] do
	datetime
	|> convert!(Calendar.ISO)
	|> to_iso8601(format)
	end
	def to_iso8601(_, format) do
	raise ArgumentError, "DateTime.to_iso8601/2 expects format to be :extended or :basic, got: #{inspect format}"
	end

Expected behavior

It would be helpful if there was an additional function before the last one to point out that the first argument is not a DateTime.

[linjunpop]

Elixir 1.5.2-otp-20 with otp 20.1:

iex(1)> DateTime.to_iso8601(NaiveDateTime.utc_now())
** (FunctionClauseError) no function clause matching in DateTime.to_iso8601/2    
    
    The following arguments were given to DateTime.to_iso8601/2:
    
        # 1
        ~N[2017-11-01 04:56:19.620912]
    
        # 2
        :extended
    
    Attempted function clauses (showing 3 out of 3):
     
        def to_iso8601(_, format) when not(format === :basic or format === :extended)
        def to_iso8601(%{calendar: Calendar.ISO, year: year, month: month, day: day, hour: hour, minute: minute, second: second, microsecond: microsecond, time_zone: time_zone, zone_abbr: zone_abbr, utc_offset: utc_offset, std_offset: std_offset}, format)
        def to_iso8601(%{calendar: _, year: _, month: _, day: _, hour: _, minute: _, second: _, microsecond: _, time_zone: _, zone_abbr: _, utc_offset: _, std_offset: _} = datetime, format)
    
    (elixir) lib/calendar/datetime.ex:329: DateTime.to_iso8601/2
iex(1)>