[Haskell-beginners] Forcing output of show

[Daniel Fischer]

On Wednesday 26 October 2011, 16:23:20, Hugo Ferreira wrote:
> Hello,
> 
> Apologies for this newbie question.
> I have a test function:
> 
> testLearnRules train = do
>    h <- IO.openFile train IO.ReadMode
>    c <- IO.hGetContents h
>    let proposedRules = instRules $ words c
>    let rs = take 10 $ M.assocs proposedRules
>    let _ = show rs
>    -- let _ = length rs
>    return ()
> 
> I realize that nothing is executed due to Haskell's
> lazy evaluation. How can I force show to print
> something to the screen?

You can't force show to rpint something, since show is a pure function not 
involving IO.
However, testLearnRules does involve IO, so instead of

    let _ = show rs

you could simply print it out,

    print rs

-- equivalently, putStrLn (show rs)

> 
> Appreciate any pointers,
> 
> TIA,
> Hugo F.