'Graceful date-parsing in Ruby

I have two date parameters in a controller action that I would like to fall-back to a default value if they are nil, or parsing fails.

Unfortunately, it seems that DateTime.strptime throws an exception if parsing fails, which forces me to write this monstrosity:

starting = if params[:starting].present?
  begin
    DateTime.strptime(params[:starting], "%Y-%m-%d")
  rescue
    @meeting_range.first
  end
else
  @meeting_range.first
end

Feels bad man. Is there any way to parse a date with the Ruby stdlib that doesn't require a begin...rescue block? Chronic feels like overkill for this situation.

rubyparsingdatetime


Solution 1:[1]

My preferred approach these days is to use Dry::Types for type coercions and Dry::Monads for representing errors.

require "dry/types"
require "dry/monads"
Dry::Types.load_extensions(:monads)
Types = Dry::Types(default: :strict)

Types::Date.try("2021-07-27T12:23:19-05:00")
# => Success(Tue, 27 Jul 2021)

Types::Date.try("foo")
# => Failure(ConstraintError: "foo" violates constraints (type?(Date, "foo"))

Solution 2:[2]

In general, I can't agree with the other solution, using rescue in this way is bad practice. I think it's worth mentioning in case someone else tries to apply the concept to a different implementation.

My concern is that some other exception you might be interested in will be hidden by that rescue, breaking the early error detection rule.

The following is for Date not DateTime but you'll get the idea:

Date.parse(home.build_time) # where build_time does not exist or home is nil
Date.parse(calculated_time) # with any exception in calculated_time

Having to face the same problem I ended up monkey patching Ruby as follows:

# date.rb
class Date
  def self.safe_parse(value, default = nil)
    Date.parse(value.to_s)
  rescue ArgumentError
    default
  end
end

Any exception in value will be rose before entering the method, and only ArgumentError is caught (although I'm not aware of any other possible ones).

The only proper use of inline rescue is something similar to this:

f(x) rescue handle($!)

Update

These days I prefer to not monkey patch Ruby. Instead, I wrap my Date in a Rich module, which I put in lib/rich, I then call it with:

Rich::Date.safe_parse(date)

Solution 3:[3]

Why not simply:

starting = DateTime.strptime(params[:starting], '%Y-%m-%d') rescue @meeting_range.first

Solution 4:[4]

All of the existing answers do have rescue somewhere. However, we can use some "ugly" methods that was available from Ruby version 1.9.3 (it was there before but there is no official description).

The method is ugly because it starts with an underscore. However, it fits the purpose.

With this, the method call in the question can be written

starting = if params[:starting].present?
  parsed = DateTime._strptime(params[:starting], "%Y-%m-%d") || {}
  if parsed.count==3 && Date.valid_date?(parsed[:year], parsed[:month], parsed[:mday])
    @meeting_range.first
  end
else
    @meeting_range.first
end
  • If the date string is matching the input format, _strptime will return a hash with all 3 date parts. so parsed.count==3 means all 3 parts exists.
  • However a further check that three parts forms a valid date in the calendar is still necessary since _strptime will not tell you they are not valid.

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Camille Goudeseune
Solution 2
Solution 3 Amadan
Solution 4 Earth Engine

Related Questions