Alexander Dunlap wrote:
On Sat, Dec 6, 2008 at 12:22 PM, Paul Johnson <[EMAIL PROTECTED]> wrote:
I've just been looking at the Data.Map function "fromListWith". According
to the docs, it has the type:
* fromListWith* :: Ord
<http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Ord.html#t%3AOrd>
k => (a -> a -> a) -> [(k, a)] -> Map
<http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map.html#t%3AMap>
k a
I'd have thought that a better type would be
* fromListWith* :: Ord
<http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Ord.html#t%3AOrd>
k => (a -> b -> b) -> [(k, a)] -> Map
<http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map.html#t%3AMap>
k b
This wouldn't break any existing code, but would allow things like
"fromListWith (:)" to do the Right Thing.
Would this be a sensible change (along with the other "with" functions in
the module).
Paul.
Hi,
I don't think that type makes sense. fromListWith takes a list of
[(key,value)] and a combining function to combine the values when
there are multiple pairs with the same key.
Ahh yes. I was thinking that the job of fromListWith was analogous to
foldr, but carrying out the fold on a per-key basis. However I see now
that it is more like foldr1 than foldr because foldr needs a zero value.
So we could have
fromListWithZero :: Ord k => (a -> b -> b) -> b -> [(k, a)] -> Map k b
fromListWithZero combiner zero pairs = ...
The first time a key is seen the combining function is called with
"zero" as its second argument. E.g.
fromListWithZero (:) [] xs
Or is that too much trouble? Accumulating a collection of lists is the
most obvious use of this function, and you can do that already (albeit
rather clunkily) with
fromListWith (++) $ map (\(k,v) -> (k, [v]) $ xs
The only time that fromListWithZero would be especially useful is when
you want the fold to be eager.
Paul.
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