Re: #4189: Add (<.>) operator (generalizing (.) to Functor)

On 02/08/10 18:44, Henning Thielemann wrote:

On Mon, 2 Aug 2010, David Menendez wrote:

...

On Sun, Aug 1, 2010 at 9:52 AM, Maciej Marcin Piechotka wrote:

...

The proposal is to add (<.>) function to Data.Functor/Control.Applicative: (<.>) :: (b -> c) -> (a -> f b) -> a -> f c f <.> g = fmap f . g -- (<.>) = (.) . fmap

In intend it is related to <$> in the same way as (.) is related to $: (a . b . c) d = a $ b $ c $ d (a <.> b <.> c) d = a <$> b <$> c <$> d

I'm not convinced. "fmap f . g" isn't that much longer than "f <.> g" and requires no new combinators.

'f' and 'g' might be infix expressions. Depending on the precedence we had to compare "fmap (f) . g" with "f <.> g" or "fmap (f) . (g)" with "f <.> g" .

Well - even for other expressions (not necessary infix: fmap (div 5) . read vs div 5 <.> read fmap (f . g) . h vs. f . g <.> h

...

I'd argue that "fmap f . fmap g . h" is better style, since it's obvious that this should be rewritten as "fmap (f . g) . h". In the example above, "a <$> b <$> c <$> d" is best transformed to "a . b . c <$> d".

I am also happy with

fmap f . fmap g . h

and

a . b . c <$> d

.

The "a <$> b <$> c <$> d" was done to show the relation between $/. and <$>/<.>. Some random usage in my files:

in Reactive e s a' (accumR' a' <.> n) r

= Reactive ef (sf <> ev <> sv) v ((`filterR` rv) <.> nf) (ff *> fv)

f `fmap` Behavior b = Behavior $ fmap f <.> b

accumB b = Behavior $ accumR <.> unBeh b

show = unsafePerformIO . (decode <.> peekArray0 0 <=< toString)

lookupQuark = guardQuark <.> flip (withArray0 0) tryString . encode

peek = Boolean . ((==0) :: GBoolean -> Bool) <.> peek . castPtr

show = unsafePerformIO . (decode <.> peekArray0 0 <=< typeName

typeFromName = typeCheck <.> flip (withArray0 0) fromName . encode

typeAncestors = unfoldr ((id &&& id) <.> typeParent)

vs.

in Reactive e s a' (fmap (accumR' a') . n) r

= Reactive ef (sf <> ev <> sv) v (fmap (`filterR` rv) . nf) (ff *> fv)

f `fmap` Behavior b = Behavior $ fmap (fmap f) . b

accumB b = Behavior $ fmap accumR . unBeh b

show = unsafePerformIO . (fmap decode . peekArray0 0 <=< toString

lookupQuark = fmap guardQuark . flip (withArray0 0) tryString . encode

peek = fmap (Boolean . ((==0) :: GBoolean -> Bool)) . peek . castPtr

show = unsafePerformIO . (fmap decode . peekArray0 0 <=< typeName)

typeFromName = fmap typeCheck . flip (withArray0 0) fromName . encode

typeAncestors = unfoldr (fmap (id &&& id) . typeParent)

Regards