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Key-value apply

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1 The problem / code

Consider the function:

apply :: (Ord k) => k -> v -> (v -> v) -> [(k,v)] -> [(k,v)]
apply k v f ds =
  let (p1,px) = span ( (k >) . fst) ds
      (p2,p3) = case px of
        []     -> ((k,v),[])
        (x:xs) -> if fst x == k
           then ((k, f $ snd x), xs)
           else ((k, v),       x:xs)
  in  p1 ++ (p2 : p3)

This takes a list of key/value pairs and processes it as follows:

  • The function is given a key to look for.
  • If the key is found, a function is applied to the associated value.
  • If the key is not found, it is inserted (at the correct place) with a specified 'default value'.
Notice that if you start with a completely empty list, you can call
several times and you will end up with a sorted list.
uses the fact that the list is sorted to cut the search short in the 'I can't find it' case - hence the

However, Haskell already provides this and much more functionality.

2 The solution:

When you are making excessive use of (key,value) pairs it is usually time to switch to
. The
function is almost the same as
, only that function has the type:
insertWith :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a

Here the update function receives the new value as well.