On Sat, Dec 20, 2008 at 6:20 PM, Brian Hurt
So, style question for people, if I can. I have a certain problem- basically, I have a bunch of functions which need a special function, of type a -> Foo say. And a bunch of other functions which can define that function on some type of interest, and then what to call the first batch of functions. I can do this either by defining a type class, something like: class Fooable a where toFoo :: a -> Foo or I can simply have all the functions which need a toFoo take an extra agrument. Performance really isn't that important here, so it's really a matter of style- which approach would people prefer in this case?
And it doesn't matter as the performance would be the same in the two cases also. My general rule of thumb is to always write combinators first, since they do not suffer the composability limitations that typeclasses do (rougly typeclasses perform a proof search which is subject to restrictions to ensure decidability, whereas with combinators you provide the proof, so there are no such restrictions). Then typeclass instances can be trivially defined in terms of the combinators. Note that the other way around is not usually possible. So eg.: module Foo where type Fooify a = a -> Foo int :: Fooify Int int = ... list :: Fooify a -> Fooify [a] list = ... -- then, if determined that this would be convenient class Fooable a where toFoo :: Fooify a instance Fooable Int where toFoo = int instance (Fooable a) => Fooable [a] where toFoo = list toFoo ... Luke
Brian
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