[Haskell-cafe] Is there a null statement that does nothing?

[Dan Weston]

If you have a long if/else if/else chain, you might consider a trivial 
case statement with guards. Whether you think this is attractive is a 
matter of taste, but it has the fall-through semantics you want and ghc 
optimizes the _ pattern matching away:

f x = case () of
   _| x == 2    -> 22
   _| x == 4    -> 44
   _| x == 7    -> 77
   _| otherwise -> 55

 > f 4
44
 > f 9
55

michael rice wrote:
> Thanks guys,
> 
> I understand what you're telling me, but have some nested IFs and just 
> want to fall through on one of the ELSES but then I end up with two 
> ELSES in a row and nothing between them. Oh, well, on to restructuring.
> 
> Michael
> 
> --- On *Wed, 10/21/09, Tim Wawrzynczak /<inforichland at gmail.com>/* wrote:
> 
> 
>     From: Tim Wawrzynczak <inforichland at gmail.com>
>     Subject: Re: [Haskell-cafe] Is there a null statement that does nothing?
>     To: "michael rice" <nowgate at yahoo.com>
>     Cc: haskell-cafe at haskell.org
>     Date: Wednesday, October 21, 2009, 8:49 PM
> 
>     Yes, an if statement must have both 'then' and 'else' branches.  As
>     an example, what if you had
> 
>     let a = if b == 2 then True else False
> 
>     and you were missing an else branch?  What would 'a' get assigned to?
> 
>     The if statement "returns" a value so must have both branches.
> 
>     However, in a monadic constraint, there are the functions 'when' and
>     'unless.'  They allow conditional evaluation of expressions in a
>     monadic context.  For example,
> 
>     main = do
>       line <- getLine
>       when (line == "hello") putStrLn "Hello back!"
> 
>     Cheers,
>      - Tim
> 
> 
>     On Wed, Oct 21, 2009 at 7:43 PM, michael rice <nowgate at yahoo.com
>     </mc/compose?to=nowgate at yahoo.com>> wrote:
> 
>         It looks like both the THEN and the ELSE in an IF expression
>         must each have an expression. What's a graceful way to do
>         nothing in either or both slots, kind of like the Fortran
>         CONTINUE statement.
> 
>           --mr
> 
>         ================
> 
>         [michael at localhost ~]$ ghci
>         GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help
>         Loading package ghc-prim ... linking ... done.
>         Loading package integer ... linking ... done.
>         Loading package base ... linking ... done.
>         Prelude> if (1==1) then else
> 
>         <interactive>:1:15: parse error on input `else'
>         Prelude> if (1==1) then True else
> 
>         <interactive>:1:24: parse error (possibly incorrect indentation)
>         Prelude> if (1==1) then True else False
>         True
>         Prelude>
> 
> 
> 
>         _______________________________________________
>         Haskell-Cafe mailing list
>         Haskell-Cafe at haskell.org </mc/compose?to=Haskell-Cafe at haskell.org>
>         http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
> 
>