[Haskell] Repeated evaluation of a constant value.

[Lemmih]

On 9/25/06, Paul Johnson <paul at 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.
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>

I've just given it a quick look so it may be flawed.

-- 
Cheers,
  Lemmih