From: Dan Weston <westondan@imageworks.com>
Subject: Re: [Haskell-cafe] Is there a null statement that does nothing?
To: "michael rice" <nowgate@yahoo.com>
Cc: "Tim Wawrzynczak" <inforichland@gmail.com>, "haskell-cafe@haskell.org" <haskell-cafe@haskell.org>
Date: Wednesday, October 21, 2009, 9:16 PM

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@gmail.com>/* wrote:
>
>
>     From: Tim Wawrzynczak <inforichland@gmail.com>
>     Subject: Re: [Haskell-cafe] Is there a null statement that does nothing?
>     To: "michael rice" <nowgate@yahoo.com>
>     Cc: haskell-cafe@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@yahoo.com
>     </mc/compose?to=nowgate@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@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>
>
>
>
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