Step by Step solution about How to resolve the algorithm Last Friday of each month step by step in the Haskell programming language

This Haskell code defines functions to find the dates for a specific weekday (Monday in this case) in a given year and then prints the results for multiple years. Here's a detailed explanation of the code:

1. Importing Modules:

   import Data.Time.Calendar
   import Data.Time.Calendar.WeekDate
   import Data.List

   This code imports necessary modules for working with dates, week dates, and list operations.

2. findWeekDay Function:

   findWeekDay :: Int -> Day -> Day

   This function takes two arguments: dayOfWeek (an integer representing the day of the week, where 1 is Monday and 7 is Sunday) and date (a Day value representing a specific date). It returns a Day value representing the date for the given dayOfWeek in the same week as the input date.

3. weekDayDates Function:

   weekDayDates :: Int -> Integer -> [String]

   This function takes two arguments: dayOfWeek (as in findWeekDay) and year (an integer representing a year). It returns a list of strings, where each string represents a date in the given year for the specified dayOfWeek.

4. Main Function:

   main :: IO ()

   This is the main function of the program.

5. mapM_ Function:

   mapM_ putStrLn

   This function applies the putStrLn function (which prints a string followed by a newline) to each element of a list. In this case, it prints each string in the list created by the following expression.

6. transpose and intercalate Functions:

   intercalate "  " <$> transpose

   The transpose function converts a list of lists into a list of lists with the elements in each inner list transposed. The intercalate function joins a list of strings into a single string using a specified separator (here, " ").

7. Generating Dates:

   (weekDayDates 5 <$> [2012 .. 2017])

   This expression generates a list of lists of strings, where each inner list contains the dates for Monday (day of the week 5) in the years 2012 through 2017.

8. Printing Results:

   mapM_ putStrLn

   This expression applies putStrLn to each list of dates, printing each list on a separate line.

import Data.Time.Calendar
       (Day, addDays, showGregorian, fromGregorian, gregorianMonthLength)
import Data.Time.Calendar.WeekDate (toWeekDate)
import Data.List (transpose, intercalate)

-- [1 .. 7] for [Mon .. Sun]
findWeekDay :: Int -> Day -> Day
findWeekDay dayOfWeek date =
  head
    (filter
       (\x ->
           let (_, _, day) = toWeekDate x
           in day == dayOfWeek)
       ((`addDays` date) <$> [-6 .. 0]))

weekDayDates :: Int -> Integer -> [String]
weekDayDates dayOfWeek year =
  ((showGregorian . findWeekDay dayOfWeek) .
   (fromGregorian year <*> gregorianMonthLength year)) <$>
  [1 .. 12]

main :: IO ()
main =
  mapM_
    putStrLn
    (intercalate "  " <$> transpose (weekDayDates 5 <$> [2012 .. 2017]))