Lackings

• IDE, non-working on vscode. Formatting, go to def, go to source, rename
• Package management, version conflicts. Easily fork & new npm pkg
• Build time slow, fragile, corruption
• Cross platform (macos win linux), works on my machine
• New dev, easily pickup setup, easily search & doc

Solved

• How to avoid Exception: empty list in real life
• Precedence in filter c `elem` ['a'..'z'] ++ ['A'..'Z'] ++ [0..9] s, and use of $
• ERR map (\xs) (subsets t) ERR map (\xs) (subsets t) FIX map (\xs -> xs) (subsets t) A value vs a function that returns a value

• Predicate and range at same time? [ x*y

  x <- [2,5,10], y <- [8,10,11], x*y > 50]

• Horrible stack build behavior - must go to global project to build to make global stack.yaml effective
• Deal with Maybe/Just
• Random IO monad: state Couldn’t match expected type ‘[a]’ with actual type ‘IO [Int]’
• do block
• error :: [Char] -> a invoke runtime/system API
• 签名结合顺序和应用顺序相反!
• Flip

  flip' :: (a -> b -> c) -> (b -> a -> c)
  flip' f = let g x y = f y x in g
  -- flip' f = g
  --   where g x y = f y x
  

TODO

• Nothing vs (), Nothing is just for Maybe? Nothing is a value constructor for Maybe type
• Type language is fragile

    data AB = A {
      a :: Int
    } | B {
      b :: Int
    } deriving (Show)
    select :: AB -> Int
    select (A a) = a
    select (B b) = b
  

• Why use let in do block? But you can’t outside
• expect Text actual String https://stackoverflow.com/questions/37894987/couldnt-match-expected-type-text-with-actual-type-char
• dot vs dollar

    putStrLn . reverseWords $ line
    putStrLn . reverseWords line -- err!
  

• Precedence? -> Biggest revelation: function is an instance of Functor typeclass!

  unwords . words <$> reverse
  unwords . reverse <$> words
  

• Use sequence for [IO Foobar] to IO [Foobar]