If its monadic code then use Control.Monad.when. If its pure code then omitting the 'else' clause makes no sense what-so-ever; to omit the else you must know the boolean is always true so why have the if? See the "Common Misunderstandings" [1] page I put together in response to seeing one too many people trip over exactly this issue.
Thomas [1] http://haskell.org/haskellwiki/Common_Misunderstandings On Wed, Oct 21, 2009 at 5:43 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 > >
_______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
