let/where

[Olaf Chitil]

Yes, Haskell is a rather big language and unfortunately has much
redundancy. It seems mostly to be a matter of taste which of `let' and
`where' you prefer. Personally, I nearly always use `where' when
possible (for the same reason you give); `let' only if the `where' would
be too far away. However, for your example I would use `case':

main = putStrLn . show $
  case maybe_read word of
    Nothing -> DP_Unkown
    Just index -> DP_Number index

(Another matter of tast: I don't like `_' in identifiers, but use
capital letters like the Haskell Prelude)

> I was disappointed to find that I don't seem to be able to write things
> like,
> 
> 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

No, the `let' is in the scope of the `where', but the `where' is not in
the scope of the `let'. Expressions are nested and allowing mutual
recursion here would make things really confusing.

Ciao,
Olaf

-- 
OLAF CHITIL, 
 Dept. of Computer Science, University of York, York YO10 5DD, UK. 
 URL: http://www.cs.york.ac.uk/~olaf/
 Tel: +44 1904 434756; Fax: +44 1904 432767