May 2021 1HaskellADay 1Liners: problems and solutions

• 2021-05-24, Monday:
  Map.partitionWithKey's discriminator is
  p :: k -> a -> Bool
  But I have a function that discriminates only on the key:
  part :: k -> Bool
  write a function that translates my discriminator that can be used by Map.partitionWithKey:
  g :: (k -> Bool) -> (k -> a -> Bool)
  • Social Justice Cleric @noaheasterly:
    g = (const .)
• 2021-05-09, Sunday:
  

  THE SEQUEL!

  Okay, kinda the same, ... but not:

  You have: f :: m a
  You want: g :: b -> m b

  Where g runs f, but accepts an argument, b.
  g drops the result of f (... on the floor? idk)
  g returns the argument b lifted to the domain, m

  GO!

  • Denis Stoyanov Ant @xgrommx:
    g (phantom . pure)
    This is just joke)
  • Social Justice Cleric @noaheasterly: (f $>)
• 2021-05-08, Saturday: two-parter
  1. You have f :: a -> m b
     You want g :: a -> m a

     That is to say: g is a function that returns the input and drops the output of f.

     so:

     blah :: (a -> m b) -> (a -> m a)
  2. What is a gooder name for the blah-function?
  • Jonathan Cast #AJAA #Resist @jonathanccast:
    returnArg = (*>) <$> f <*> return
  • Social Justice Cleric @noaheasterly:
    liftA2 (<*)