Hello michael, Thursday, October 22, 2009, 4:59:43 AM, you wrote:
"return ()" does the trick if another branch also returns () > 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 <[email protected]> wrote: > From: Tim Wawrzynczak <[email protected]> > Subject: Re: [Haskell-cafe] Is there a null statement that does nothing? > To: "michael rice" <[email protected]> > Cc: [email protected] > 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 <[email protected]> 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 > ================ > [mich...@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 > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Best regards, Bulat mailto:[email protected] _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
