On 6/24/12 5:41 AM, Anton Kholomiov wrote:
I think monad's methods are misleading, let's rename them
class Monad m where
idM :: a -> m a
(*$) :: (a -> m b) -> m a -> m b
We can see that `return` is a monadic identity and the `bind`
is an application in disguise. So now we have two applications.
It's standard `($)` and monadic `(*$)`. But they are application.
Well isn't it something like `plusInt` and `plusDouble`?
Maybe we can devise single class for application. Let's
imagine a special class `App`
class App ?? where
($) :: ???
As you can see it's defined so that we can fit
monads and plain functions in this framework. Moreover
if we redefine this class than whitespace is redefined
automatically!
The class you're looking for is Applicative. The (<*>) operator handles
application of "effectful" things to "effectful" things, whereas (<$>)
handles the application of non-"effectful" things to "effectful" things.
This situation is interesting because it highlights the fact that there
is a distinction between the meaning of whitespace between function and
argument vs the meaning of whitespace between argument and argument.
The desire to invoke (<$>) and (<*>) implicitly is known as idiom
brackets. SHE provides this as syntactic sugar, and of course you can
define your own version using iI and Ii as your "brackets".
So `($)` really means *white space* in haskell.
Not entirely so, as noted above. Though yes, ($) denotes application in
just the same way as function--argument whitespace does. However, the
presence of ($) as an operator is helpful because it allows us to
capture the syntax of our language, as in:
fmap ($x) fs
zipWith ($) fs xs
...
--
Live well,
~wren
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