[Haskell-cafe] lazily-loaded structures
pgsql at j-davis.com
Fri Jan 1 17:17:09 EST 2010
I'm relatively new to haskell.
Let's say I have a simple program like this:
type Tree = String
type StringMap = Map String String
f :: StringMap -> Tree -> Tree
f _ y = y ++ "!"
getStringMap :: IO StringMap
getStringMap = return (Data.Map.fromList [("k1","v1")])
main :: IO ()
main = do
m <- getStringMap
s <- getLine
putStr $ show (f m s) ++ "\n"
Ignore the implementation of f and getStringMap for now. Assume f is a
fairly complex function that uses the Map during the process of
transforming the Tree argument into the result Tree (here I defined Tree
as a string just so that it would compile).
getStringMap reads the whole Map into memory at once. However, now let's
say that the whole Map is too large, so I decide to store it over a
network in a remote database system instead. And then I define another
type Connection = String
getStringMapValue :: Connection -> String -> IO String
that takes a connection string and a key and returns the value.
My question is this: how do I change my program to make StringMap a
lazily-loaded structure based on getStringMapValue without changing f?
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