[Haskell-cafe] commenting out a line of code

Bernhard Herzog bh at intevation.de
Tue Nov 15 16:21:51 UTC 2016

On 15.11.2016, David Turner wrote:
> The outer do block should be desugared first, and the `putStrLn "done"` has
> the same indent as the `if` so it gets a semicolon, in that block, meaning
> it looks like this:
> main = do
>   { if True
>     then do
>       putStrLn "A"
>     else do
>       -- putStrLn "B"
>   ; putStrLn "done"
>   }

That's what one would expect. Except that GHC treats the 'putStrLn "done"' as 
part of the do-block in the else branch by default. This is due to the 
NondecreasingIndentation extension which is enabled by default (see 
With the option -XNoNondecreasingIndentation (or -XHaskell2010) GHC does 
produce an error message because of the empty do-block.


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