# Key-value apply

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Revision as of 18:36, 16 February 2007 by BrettGiles (talk | contribs) (Clean up to make more like a how-to, move context to talk page)

## 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 `apply`

several times and you will end up with a sorted list. `apply`

uses the fact that the list is sorted to cut the search short in the 'I can't find it' case - hence the `Ord`

context.

However, Haskell already provides this and much more functionality.

## The solution: `Data.Map`

When you are making excessive use of (key,value) pairs it is usually time to switch to `Data.Map`

. The `apply`

function is almost the same as `Data.Map.insertWith`

, 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.