Am Dienstag 16 September 2003 04:57 schrieb Yu Di:
Hi, I want to create an arrow which is essentially
data MyArrow a b = MyArrow ((String, a) -> (String,b))
i.e. there is an "information" asscioated with each piece of data (represented by the string), and I want to pass it around. But I have a problem about how to define "pure" and "first". At first, I declared
pure f = MyArrow (\(s, x) -> (s, f x)) first (MyArrow f) = MyArrow (\(s, (x, y)) -> let (s', z) = f (s, x) in (s', (z, y)))
this seems to work, but then I begin to have problems with the "data-plumbing" pure arrows, e.g. in
pure (\x -> (x, x)) >>> first someArrow >>>> pure (\(_, x) -> x)
Ideally, this arrow will preserve whatever information I put there for the input, but because "first someArrow" will change the whole information associated with the pair of result, I can't find any way to let "pure (\(_, x)->x)" (which is an extremely generic function) retrieve the part of information for the second piece in the pair tuple.
what does the compiler say? or is it a runtime error? how did you implement "(>>>) :: a b c -> a c d -> a b d"? (MyArrow f1) >>> (MyArrow f2) = MyArrow (f2 . f1) does this compile?: (pure (\x -> (x, x)) :: MyArrow a (a,a)) >>> (first (someArrow :: MyArrow a b) :: MyArrow (a,a) (b,a)) >>> (pure (\(_, x) -> x) :: MyArrow (b,a) a) pure and first seem to be correct. but ... just as an (slow) alternative: -- first :: a x fx -> a (x, y) (fx, y) first (MyArrow f) = MyArrow $ (\((fs,fx),y)->(fs,(fx,y))) . (\(sx,y)->(f sx,y)) . (\(s,(x,y))->((s,x),y)) - marc
Of course I can create specialized arrows for the tasks \x -> (x, x) and \(_, x) -> x which passes the information around, but this will become tedious as I will have to define specialized arrows for a lot of similar tasks one by one, and I won't be able to use the arrow pre-processor at all.
So how can I implement this? Thanks very much!
Di, Yu 9.15
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