On 8/13/09, Jake McArthur
Jake McArthur wrote:
The monoids package offers something similar to this:
mapReduce :: (Generator c, Reducer e m) => (Elem c -> e) -> c -> m
If we take (Elem c) to be (item), (e) to be (item'), (c) to be (full), and (m) to be (full'), it's basically the same thing, and offers the same advantages as the ones you listed, as far as I can tell.
Your example about uvector inspired me to try writing out the necessary instances for uvector:
instance UA a => Monoid (UArr a) where mempty = emptyU mappend = appendU
instance UA a => Reducer a (UArr a) where unit = singletonU snoc = snocU cons = consU
instance UA a => Generator (UArr a) where type Elem (UArr a) = a mapTo f = foldlU (\a -> snoc a . f)
This looks to be essentially the same as the 'map' function in ListLike, and suffers from the same problem. It won't have the performance characteristics of the native map functions. Using e.g. ByteStrings, you're recreating a ByteString by snoc'ing elements. This might work with UVector (I intend to try it this evening); I don't know how well the fusion framework will hold up in class dictionaries. Still, the monoids package is very powerful (and I'd completely forgotten it). Perhaps there's another approach that would work? John