Question

Here is a function that doesn’t sit right with me, I’m thinking I need to get rid of the fst.

flagHolidays :: [(C.Day,Availability)] -> Handler [(C.Day,Availability)]
flagHolidays dayPairs = do
   let days = map fst dayPairs
       yepNope = Prelude.map isHoliday days
       availability = Prelude.map flagAvailability yepNope
   return $ Prelude.zip days availability

The only way I can think to do it is a very ugly pattern match, along the lines of (x,y):(xs,ys). Does the removal of fst make sense? If so, what’s the best way to pattern-match this list of pairs?

Answer 1

Make the function pure and

flagHolidays = map (second $ flagAvailability . isHoliday)

And to answer the actual question, unless passed as an argument fst and snd are rarely used.

[Edit] Hmmm, I was a bit hasty writing that. It’s wrong. You’re passing in a list of pairs and never use the second component of the pair. Corrected, but in the same vein (and now using fst):

flagHolidays = map ((id &&& flagAvailability . isHoliday) . fst)