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Wed Feb 24 21:16:14 CET 2010

## Applicative programming with effects.

I'm reading [1] again. It's about ``pure functions applied to funny
arguments.''
Some aha's of a concrete-minded Schemer:
* map in Scheme is map, zipWith, zipWith2, ... in Haskell.
These need to be different functions as they have different
type signatures.
* The S and K combinators are `ap' and `return' from the environment
(reader) monad. The paper says that S & K are ``designed for this
purpose''. That's the first time I hear this. But surely, looking
at S indeed it applies proto-function and proto-argument to an
environment, and applies the resulting function to the resulting
argument.
Now the concrete-minded mind needs to take a distance from looking at
a functor as a data structure over which one maps a function
piecewize, and instead see it as a computation. Best to start with
monads, as each monad is a AF.
What I don't quite get is this idea of "pure function & effects" where
the <*> operator combines effects and the `pure' operator lifts a pure
function into the effectful domain.
Let's start with
sequence :: (Monad m) => [m a] -> m [a]
just as in the paper. This function takes a list of computations and
produces a list of results, threading the monadic effect.
sequence [] = return []
sequence (c:cs) = do
x <- c
xs <- sequence cs
return (x:xs)
Which can be written differently as:
sequence [] = return []
sequence (c:cs) = pure (:) <*> c <*> sequence cs
The paper then generalizes this to `traverse'. The key point being
that the recursive call is _inside_ the effectful world.
Now I can't bridge this explanation with the type signature of an
applicative functor:
pure :: x -> a x
(<*>) :: a (x->y) -> a x -> a y
Probably for the same reason that I couldn't see this for Monads in
the beginning. My intuitive confusion there was that I was looking at
`a' as a data constructor and a type constructor at the same time.
To state the (now) obvious: The two lines above are part of a class
definition `Applicative a', where `a' is a type variable of kind * ->
*, I.e. a parameteric type with one parameter. The `Applicative' is
(like) a predicate on type variables.
[1] http://www.cs.nott.ac.uk/~ctm/IdiomLite.pdf

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