Re: [Haskell-cafe] What is this applicative functor?

Ah right, I didn't stare hard enough at the Applicative. If I squint a bit at the results of your Applicative instance it seems similar to the difference between []'s Applicative and the ZipList Applicative, except a little crossed up. Sure you already identified that, sorry I don't have more to offer here. On Tue, Jan 31, 2017 at 2:36 PM, Joachim Breitner wrote:

Hi,

good idea, but no. The datatypes are superficially equivalent, but the Applicative instance differs:

With NonEmpty’s instance:

...

(,) <$> 0 :| [1,2] <*> 3 :| [4,5] (0,3) :| [(0,4),(0,5),(1,3),(1,4),(1,5),(2,3),(2,4),(2,5)]

with my instance:

...

(,) <$> 0 :| [1,2] <*> 3 :| [4,5] (0,3) :| [(1,3),(2,3),(0,4),(0,5)]

Greetings, Joachim

Am Dienstag, den 31.01.2017, 14:24 -0600 schrieb Christopher Allen:

...

NonEmpty?

On Tue, Jan 31, 2017 at 2:22 PM, Joachim Breitner 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@joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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

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-- Chris Allen Currently working on http://haskellbook.com