On 9/25/06, Paul Johnson <paul@cogito.org.uk> wrote:
I recently wanted to pass a lookup table around a program. Rather than having something explicitly typed as Map.Map I wrote a function that assembled a Map and then used Map.lookup to return a function, like this:
module Main where
import qualified Data.Map as Map import Debug.Trace
alist :: [(String, Integer)] alist = map (\i -> (show i, i)) [1..100]
Change:
table :: [(String, Integer)] -> String -> Maybe Integer table ls str = Map.lookup str fm where fm = trace "Trace: making the map" $ Map.fromList ls
to:
table :: [(String, Integer)] -> String -> Maybe Integer table ls = \str -> Map.lookup str fm where fm = trace "Trace: making the map" $ Map.fromList ls
"table" can be seen as a function from an association list to a look-up function.
demo :: [(String, Integer)] -> IO ()
Change:
demo ls = do
to:
demo ls = func `seq` do
showLookup "5" showLookup "70" showLookup "164" showLookup "wibble" where func = table ls showLookup str = putStrLn $ "Look up " ++ show str ++ " gives " ++ show (func str) ++ "."
main :: IO () main = demo alist
I reasoned that closure returned by "table ls" would contain a thunk for "fm", which in turn would be evaluted the first time it was called. But it isn't: instead "fm" gets evaluated for every call to "table", as shown by the repeated trace messages.
Store this message as "Thunk.lhs". Compile with ghc -O2 and run "main". What you get is:
Trace: making the map Look up "5" gives Just 5. Trace: making the map Look up "70" gives Just 70. Trace: making the map Look up "164" gives Nothing. Trace: making the map Look up "wibble" gives Nothing.
So my question is: how do I write "table" to return a function which does not build the lookup table for every call? Or is this a bug in GHC?
(I should add that my real "table" function is rather more complicated, and only invokes Map.lookup on a subset of its arguments.)
Paul. _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
I've just given it a quick look so it may be flawed. -- Cheers, Lemmih