let/where

[Ian Lynagh]

On Wed, Sep 19, 2001 at 01:53:22PM -0400, Mark Carroll wrote:
> 
> main = let maybe_index = maybe_read word 
>        in putStr (show (if maybe_index == Nothing then DP_Unknown else DP_Number index) ++ "\n")
>           where (Just index) = maybe_index
> 
> BTW, is the above a sane way of getting the 'index' 'out of' the "Just"?
> I often seem to be using a "where (Just foo) = bar" type of idiom.

I'd probably write this as something like

main = case maybe_read word of
           Just index -> putStrLn $ show $ if maybe_index == Nothing
                                           then DP_Unknown
                                           else DP_Number index
           Nothing -> error "No index found"

As for let versus where, I use whatever I think looks best, which tends
to be where unless I have lots of nested ifs and cases.


Ian