[Haskell-cafe] What is this applicative functor?

Joachim Breitner mail at joachim-breitner.de
Tue Jan 31 20:22:35 UTC 2017


I recently wrote this applicative functor:

    data OneStep a = OneStep a [a]

    instance Functor OneStep where
        fmap f (OneStep o s) = OneStep (f o) (map f s)

    instance Applicative OneStep where
        pure x = OneStep x []
        OneStep f fs <*> OneStep x xs = OneStep (f x) (map ($x) fs ++ map f xs)

    takeOneStep :: OneStep t -> [t]
    takeOneStep (OneStep _ xs) = xs

This was useful in the context of writing a shrink for QuickCheck, as
discussed at http://stackoverflow.com/a/41944525/946226.

Now I wonder: Does this functor have a proper name? Does it already
exist in the libraries somewhere? Should it?


Joachim “nomeata” Breitner
  mail at joachim-breitner.dehttps://www.joachim-breitner.de/
  XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
  Debian Developer: nomeata at debian.org
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