[Haskell-cafe] commenting out a line of code

[Doaitse Swierstra]

The answer is very simple ;-} The report says that after a do the first symbol to encounter determines the indentation level; so putStrLn “done” is part of the else branch. Obvious, isn’t it.

Se also https://wiki.haskell.org/Common_Misunderstandings

 Doaitse


> Op 15 nov. 2016, om 13:31  heeft Norbert Melzer <timmelzer at gmail.com> het volgende geschreven:
> 
> I'm not quite sure, but I'd guess either one of
> 
> 1.) It does not compile at all because of suspicious indentation
> 2.) It will just print "A" but not "done" because that is considered beeing in the else-branch.
> 
> Johannes Waldmann <johannes.waldmann at htwk-leipzig.de <mailto:johannes.waldmann at htwk-leipzig.de>> schrieb am Di., 15. Nov. 2016 um 13:11 Uhr:
> Dear Cafe -
> 
> Sometimes, a comment can change the meaning of a program.
> No? Well, but "commenting out" a line of code certainly does.
> So, can you guess what this program will output?
> 
> main = do
>   if True
>     then do
>       putStrLn "A"
>     else do
>       -- putStrLn "B"
>   putStrLn "done"
> 
> I nominate this for inclusion in a hypothetical
> Haskell equivalent of http://www.javapuzzlers.com/ <http://www.javapuzzlers.com/>
> 
> - J.
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