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

michael rice nowgate at yahoo.com
Wed Oct 21 20:59:43 EDT 2009

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.


--- 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!"

 - Tim

On Wed, Oct 21, 2009 at 7:43 PM, michael rice <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.



[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


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