On Tue, Jan 14, 2014 at 3:30 PM, Christian Marie wrote:

On Tue, Jan 14, 2014 at 04:57:14PM +1000, Tony Morris wrote:

...

Why not generalise to any functor? let flip f a = fmap ($a) f

I didn't think of that, I suppose.

Now that I do, it seems to be a tradeoff between (arguably) less immediately obvious type signatures and something.

I can't actually think of what that something is yet. Can you think of a real world use for a functor flip where normal flip wouldn't do?

I know I've written "fmap ($a) f" in the past, so I guess any of those would be a use case for a functor flip. On the other hand, it's only a few characters longer than flip, and arguably more clear, so I'm not convinced that generalizing flip is useful. YMMV.

While I don't think the overall idea is good*, I do wonder if the explicit numbers in the names could be removed. Here are some approaches to writing N-ary zipWith that might give you inspiration: https://gist.github.com/dagit/6082516 https://gist.github.com/dagit/8428444 Both have links to the original sources. The first one depends on closed type families and the other one still uses numerals but they are just names for special functions that makes it all hold together. * I don't think we should encourage use of things like flip8 in 'real code' because it's very likely to make the code completely incomprehensible. I DO think it's a fun and interesting exercise to generalizing flip to N-ary. Jason

On Mon, Jan 13, 2014 at 10:57 PM, Tony Morris wrote:

On 14/01/14 15:48, Christian Marie wrote:

I have defined a bunch of functions like this:

-- | Move the fourth argument to the first place rotate4 :: (a -> b -> c -> d -> e) -> (d -> a -> b -> c -> e)

-- | Reverse four arguments flip4 :: (a -> b -> c -> d -> e) -> (d -> c -> b -> a -> e)

I decided to upload this as a library to hackage, as I personally find it useful, especially for writing FFI bindings.

It seems like I can't be the first to write a library like this though, am I missing something obvious? Is this useful or stupid? Does it exist already?

Full definition here: https://github.com/christian-marie/flippers/blob/master/src/Control/Flippers...

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Why not generalise to any functor?

let flip f a = fmap ($a) f

I don't see that operator as "flip-like", personally. It might behave as flip in the case of the function functor , but that intuition does not carry over to things like fmap ($a) [f, g, h] = [f a, g a, h a] I'd call it "funder", personally.

Indeed! I offered the question as a coaching exercise :) On 15/01/14 19:56, Tom Ellis wrote:

On Tue, Jan 14, 2014 at 04:57:14PM +1000, Tony Morris wrote:

...

Why not generalise to any functor?

let flip f a = fmap ($a) f FYI, this is Control.Lens.??

http://hackage.haskell.org/package/lens-3.9.2/docs/src/Control-Lens-Combinat...

Tom

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