Difference between revisions of "Key-value apply"

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(Why we did this.)
(Clean up to make more like a how-to, move context to talk page)
 
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I just wrote this function:
 
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==The problem / code ==
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Consider the function:
   
 
<haskell>
 
<haskell>
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</haskell>
 
</haskell>
   
As you can see (?!), this takes a list of key/value pairs and processes it as follows:
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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. (Note that <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.)
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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.
   
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 '<hask>apply</hask>'? Have you ever had call to use such a function yourself?
 
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However, Haskell already provides this and much more functionality.
   
== Data.Map ==
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== 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>. Your <hask>apply</hask> function is almost the same as <hask>Data.Map.insertWith</hask>, only that function has the type:
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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.
--[[User:Twanvl|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!
 
 
[[User:MathematicalOrchid|MathematicalOrchid]] 19:27, 15 February 2007 (UTC)
 
 
== Outer Context ==
 
 
This function actually occurred in the definition of a larger utility. It has the type
 
 
<haskell>
 
decode :: (Eq k) => [([k],[v])] -> [k] -> [v]
 
</haskell>
 
 
This takes a large input, searches the lookup table for the longest possible matching key, and returns the corresponding value. It then processes the rest of the input the same way.
 
 
The <hask>apply</hask> function above occurs because the case actually transforms the <hask>[([k],[v])]</hask> into a ''tree'' to facilitate faster searching. (I gather that <hask>Data.Map</hask> does the exact same thing - but it's already implemented for you!)
 
 
Is a function like <hask>decode</hask> already out there somewhere?
 
   
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[[Category:How to]]
 
[[Category:Code]]
 
[[Category:Code]]

Latest revision as of 18:36, 16 February 2007

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.