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]
>
> 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 ()
> demo ls = 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.
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