Olaf Chitil olaf@cs.york.ac.uk
Wed, 19 Sep 2001 19:33:12 +0100

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.


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