[Haskell-cafe] Re: not possible with monad transformers ?

[Tomasz Zielonka]

On Tue, Nov 30, 2004 at 06:36:46PM +0000, Pavel Zolnikov wrote:
> The problem is that code on line 4 is useless.  
> If one of the arguments a or b is Nothing, computation will just return 
> Nothing and will not bother to execute lines 4-5:
> 
> 1   foo a b = do
> 2      output "before" 
> 3      let r = liftM2 (+) a b 
> 4      when (r == Nothing) $ output "error" 
> 5      return r -- ??? "lift r"

Obviously you are talking about your version of code (with "r <- liftM2..."),
because for Mike's code your statement isn't true. How can "let" abort the
computation?

Best regards,
Tomasz