Difference between revisions of "Key-value apply"
(Data.Map can be used) |
(Nice one...) |
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== Data.Map == |
== Data.Map == |
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− | When you are making excessive use of (key,value) pairs it is usually time to switch to Data.Map. <hask>apply</hask> is almost the same as <hask>Data.Map.insertWith</hask>, only that function has the type: |
+ | When you are making excessive use of (key,value) pairs it is usually time to switch to <hask>Data.Map</hask>. Your <hask>apply</hask> function is almost the same as <hask>Data.Map.insertWith</hask>, only that function has the type: |
<haskell> |
<haskell> |
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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 |
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Here the update function receives the new value as well. |
Here the update function receives the new value as well. |
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--[[User:Twanvl|Twanvl]] |
--[[User:Twanvl|Twanvl]] |
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+ | - |
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+ | Thanks for the tip! A whole new module for me to learn. My oh my... I do love the way Haskell type signatures almost tell you what the whole function does! |
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+ | [[User:MathematicalOrchid|MathematicalOrchid]] 19:27, 15 February 2007 (UTC) |
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[[Category:Code]] |
[[Category:Code]] |
Revision as of 19:27, 15 February 2007
I just wrote this 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)
As you can see (?!), 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. (Note that 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.)
Does a function like this already exist somewhere? (Hoogle seems to indicate not.) Is this a special case of something more general? Is there a better implementation? (The code isn't very readable at it is.) Can you think of a better name than just 'apply
'? Have you ever had call to use such a function yourself?
Data.Map
When you are making excessive use of (key,value) pairs it is usually time to switch to Data.Map
. Your 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. --Twanvl
-
Thanks for the tip! A whole new module for me to learn. My oh my... I do love the way Haskell type signatures almost tell you what the whole function does!
MathematicalOrchid 19:27, 15 February 2007 (UTC)