Am Samstag 21 November 2009 21:33:28 schrieb Philip Scott:
Hi ho,
In general, however, you just need practice. Go code! =)
Righto, I am getting stuck in with that. One last question; I've been trying to read up on Arrows and my mind is being boggled. Via experiment, I have worked out what 'second' was doing (the documentation is useless unless you already understand a lot of stuff I clearly don't)
For the other newbies, 'second' takes a function and a tuple, it applies the function to the second thing in your tuple and returns a tuple with the first
value unchanged, and the result of applying 'f' to the second:
second (\x -> "fish") (10,20)
(10,"fish")
What I am struggling to understand is what on earth the type signature means: :t second
second :: (Arrow a) => a b c -> a (d, b) (d, c)
How can (\x -> "fish") be an 'a b c' when it really looks like this: :t (\x->"fish") (\x->"fish") :: t -> [Char]
a is a type variable (restricted to be a member of the Arrow class). Now the type ghci reports for (\x -> "fish") is printed in infix form, in prefix form, it reads :t (\x -> "fish") (\x -> "fish") :: (->) t [Char] so we find a = (->) b = t c = [Char] and you're using the most widespread instance of Arrow, (->). Arrows are a generalisation of functions. Until you're more familiar with Arrows, I suggest replacing any (Arrow a) with (->) in the type signatures to understand what things mean in the familiar case. Next in line would probably be Kleisli arrows (Monad m => a -> m b; it's wrapped in a newtype for Control.Arrow), break at any level of abstraction you want and return later.
And I am pretty sure I never made any Arrpws...
There are a few others have made for you to use :)
I feel I am on the verge of understanding something deep and fundamentally philosophical about the typesystem but I can't quite bend my mind around to it
:)
All the best,
Philip