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module Y2017.M01.D19.Solution where
import Data.Monoid hiding (First)
-- below imports available via 1HaskellADay git repository
import Y2017.M01.D16.Solution
And lastly from @Jose_A_Alonso @yoeight, lastest, in the theme of Traversable.
So we've seen the First / find pairing (see import above).
But how do we get the 'last' element of a Traversable type? What does that
even mean?
Today's Haskell exercise: define lastest.
-- so, from @yoeight we have First, let's do the Dual of that ... which is
-- part of Data.Monoid
*Y2017.M01.D19.Solution> Dual (First (Just 'X')) <> Dual First (Const (Just 'Y'))
Dual {getDual = Const {val = Just 'Y'}}
lastest :: (Traversable t) => t a -> Maybe a
lastest = first . getDual . foldMap (Dual . First . Just)
-- Question: Is lastest a total function? Yes. Let's see:
nada, uno, multa :: [Int]
nada = []
uno = [1]
multa = [1..10]
What is the lastest for each of these Traversables: nada, uno, multa?
*Y2017.M01.D19.Solution> map lastest [nada, uno, multa]
[Nothing,Just 1,Just 10]
-- Tomorrow, we'll look at making Merkle trees Traversable and see what
-- that means for these functions we've defined.