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 > > _________________________________________________________________ _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
