Is there a function in Haskell that will allow me to enter component of a date (like a string representation or day month year components) that I can get information from (like day of week, days in a month, etc.)?

I've looked online and it looks like there are a lot of custom libraries, but I'm hoping there's one in the standard prelud开发者_如何学JAVAe library of ghci 10.6.4 that's just not well documented?

Are Data.Time.Calendar and Data.Time.Format in the time library sufficient?

You can parse a string representation of a date and get the length of a month using gregorianMonthLength. Not sure about day of the week, though you could format the date as a string using a format that just displays the week day.

A quick Google search turns up this, which may be what you want. It lets you parse strings representing dates and extract information from them.

You can find the day of the week with mondayStartWeek or sundayStartWeek, depending on whether you think a week starts on Monday, or on Sunday. Both functions are in Data.Time.Calendar.OrdinalDate.

λ> snd $ mondayStartWeek $ fromGregorian 2017 10 3
2

In the above example, the return value is 2, which indicates the second day of the week. Since the function is called mondayStartWeek, Monday is the first day, so 2 corresponds to Tuesday. This is true of October 3, 2017.

A warning regarding week numbers

Both functions return a tuple, where the second element is the week day. As far as I can tell, that should be trustworthy.

The first element, however, is the week number of the year. Be careful with that, because the rules for week numbering are political. If I remember correctly, in USA, week 1 is the week that contains January 1. That's not the case in Denmark, where I live. Here, week 1 is the first week where Thursday falls in the new year. This can mean that December 31 can fall in week 1 of the next year. IIRC, this is the rule for many other European countries. Some years, the American and the European week numbers align, but some years, they don't.