begin Vo Minh Thu quotation:
> I guess it is short because you make use of second... so you can
> define second' for your B data type, or make B an instance of Arrow.
I don't think that's the case. The code for "f" is making use of the
Arrow instance for (->):
second :: Arrow a => a b c -> a (d, b) (d, c)
(str ++) :: [Char] -> [Char]
second (str ++) :: (d, [Char]) -> (d, [Char])
All the caller can control here is what sort of "d" is passed through
unchanged, not the fact that the resulting function expects a pair and
returns a pair.
-md
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