Posts

Showing posts from October 2, 2016

TMTOWTDI, Haskell Style

I assure you there will be no further allusions to Korean earworms. That said, on to the subject at hand...

Remember the exercise in the online Haskell course that had several tests to filter out weak passwords, all of which had to pass for the fictitious system to allow a String value to be used as a password? I wanted to make it easy to change, so I wanted to take a [String -> Bool] and get a [Bool] I could apply and to for the final thumbs up/thumbs down decision.

The first step: roll my own, which has a pleasing symmetry with map if you write it as a list comprehension:

wonkyMap fs x = [f x | f <- fs]

Then I stumbled across Derek Wyatt's blog post about using sequence for the purpose. Life was good... and then I got Haskell Programming from first principles, and life got better, because its authors do a very good job of explaining the Applicative type class. Applicative defines two functions, pure and (<*>):

class Functor f => Applicative f where
    pure  :: a -&g…