# Key-value apply

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− | + | ==The problem / code == | |

+ | Consider the function: | ||

<haskell> | <haskell> | ||

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</haskell> | </haskell> | ||

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

* The function is given a key to look for. | * The function is given a key to look for. | ||

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* If the key is ''not'' found, it is inserted (at the correct place) with a specified 'default 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 <hask>apply</hask> several times and you will end up with a sorted list. | + | Notice that if you start with a completely empty list, you can call <hask>apply</hask> several times and you will end up with a sorted list. <hask>apply</hask> uses the fact that the list is sorted to cut the search short in the 'I can't find it' case - hence the <hask>Ord</hask> context. |

− | + | However, Haskell already provides this and much more functionality. | |

− | == Data.Map == | + | == The solution: <hask>Data.Map</hask> == |

− | When you are making excessive use of (key,value) pairs it is usually time to switch to <hask>Data.Map</hask>. | + | When you are making excessive use of (key,value) pairs it is usually time to switch to <hask>Data.Map</hask>. The <hask>apply</hask> function is almost the same as <hask>Data.Map.insertWith</hask>, only that function has the type: |

<haskell> | <haskell> | ||

insertWith :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a | insertWith :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a | ||

</haskell> | </haskell> | ||

Here the update function receives the new value as well. | Here the update function receives the new value as well. | ||

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+ | [[Category:How to]] | ||

[[Category:Code]] | [[Category:Code]] |

## Latest revision as of 18:36, 16 February 2007

## [edit] 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'.

apply

apply

Ord

However, Haskell already provides this and much more functionality.

## [edit] 2 The solution: Data.Map

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

Data.Map

apply

Data.Map.insertWith

insertWith :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a

Here the update function receives the new value as well.