Claus Reinke wrote:
mapA f (nilAP -> ()) = nilA
mapA f (consAP -> (h,t)) = consA (f h) (mapA f t)
foldA f n (nilAP -> ()) = n
foldA f n (consAP -> (h,t)) = f h (foldA f n t)
To me this exactly illustrates why view patterns are a bad idea: you've
taken some concrete type, abstracted it to replace the actual structure by a
list structure, then defined map and fold over the list structure. This
means that map and fold can't take advantage of the actual concrete
structure and are therefore condemned to use the inefficient linear
structure imposed by the list abstraction.
For example implementing map over a tree directly, gives the possibility of
parallel execution since different subtrees can be mapped independently. But
when you view the tree abstractly as a list, no such parallel execution can
take place. Therefore surely it is better that map and fold are defined for
each ADT separately, with the separate definitions hidden behind a type
class, than to attempt to define them "outside" the definition of the ADT
using view patterns?
Brian.
--
http://www.metamilk.com
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