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.
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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