[Haskell-cafe] What is this applicative functor?

[Christopher Allen]

NonEmpty?

On Tue, Jan 31, 2017 at 2:22 PM, Joachim Breitner
<mail at joachim-breitner.de> wrote:
> Hi,
>
> 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?
>
> Greetings,
> Joachim
>
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
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-- 
Chris Allen
Currently working on http://haskellbook.com