Haskell One-liners

One-liners
  • 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(?)

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