Monday, March 13, 2017

January 2017 1HaskellADay 1Liners

  • January 31st, 2017:
    You have d = "3.461497957769017000D+07"
    define parseDouble :: String -> Double
    (n.b.: (read d) :: Double throws an error)
    • bazzargh @bazzargh uncurry (*) (bimap read ((10^).read.(dropWhile(`elem`"0+D"))) (break (=='D') d))::Double
  • January 31st, 2017: Given
    e :: FilePath -> [String] -> [Epoch]

    readEpochs :: FilePath -> IO [Epoch]
    readEpochs f = e f . lines <$> readFile f

    point-free-itize readEpochs
    • Astynax Pirogov @alex_pir uncurry fmap . (((. lines) . e) &&& readFile
  • January 30th, 2017: Given

    parseNum :: String -> Maybe (Float, String)

    define: dropNum'' :: String -> Maybe String

    points-free in terms of parseNum
    • matt @themattchan dropNum" = fmap snd . parseNum
  • January 30th, 2017: For

    parseHeader :: String -> Maybe String
    parseHeader str = match "Start " str <|> match "Finish " str

    eliminate redundancies
    • mconcat . ([match] <*> ["Start ", "Finish "] <*>) . pure
    • Nickolay Kudasov @crazy_fizruk If you're allowed to use Monoid instead of Alternative, how about this version?
      foldMap match ["Start", "Finish"]
    • Andreas Källberg @Anka213  My solution was
      parseHeader str = foldr1 (<|>) . map (`match` str) $ ["Start", "Finish"]
      But that's longer than the original.
  • January 25th, 2017:
    given f is type: f :: Int -> a -> Bool

    for: g :: a -> Bool
    g = (||) . f 2 <*> f 27

    rewrite g using f only once in the definition
    • Denis Stoyanov @xgrommx ugly version but
      (liftA2 . liftA2) (||) ($2) ($27) f
  • January 19th, 2017:
    import Data.Tree.Merkle

    mkleaf :: Show a => a -> Leaf a
    mkleaf = uncurry Leaf . (show . hashDatum &&& id)

    redefine using (<*>)
    • Denis Stoyanov @xgrommx smth like
      mkleaf = uncurry Leaf . show . (hashDatum <$> (,) <*> id)
      mkleaf = uncurry Leaf . show . (liftA2 (,) hashDatum id)
  • January 19th, 2017:
    mkbranch1 :: Leaf a -> Branch a
    mkbranch1 = uncurry Twig . (uncurry childrenHash . (dataHash &&& dataHash) &&& id)

    redefine using (<*>)s(?)

1 comment:

Mahalya sree said...

All of these tips are great, that’s very interesting. I’m so tempted to try that myself, but you would think if it were effective, more people would do it.
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