I havve just tested this with ghc8, and surprisingly when B is commented out the last putStrLn is treated as a part of if-else. So, with B commented out the only thing printed out is A and not 'done'.

From: David Turner Sent: Tuesday, November 15, 2016 1:14 PM To: Haskell Cafe Subject: Re: [Haskell-cafe] commenting out a line of code

On 15 November 2016 at 12:53, Doaitse Swierstra <doaitse@swierstra.net> wrote:

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.

 

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"

  }

On 15 November 2016 at 12:31, Norbert Melzer <timmelzer@gmail.com> wrote:

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.

I get *both* of these behaviours! When building, it complains about an empty 'do' block. When running through `stack ghci` it doesn't seem to mind.

[linuxadmin@trac-leeds-09-vm01 13:07:57 ~/indent-puzzle ]

$ stack build

indent-puzzle-0.1.0.0: build

Preprocessing executable 'indent-puzzle-exe' for indent-puzzle-0.1.0.0...

[1 of 1] Compiling Main             ( app/Main.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/indent-puzzle-exe/indent-puzzle-exe-tmp/Main.o )

/home/linuxadmin/indent-puzzle/app/Main.hs:5:10: Empty 'do' block

--  While building package indent-puzzle-0.1.0.0 using:

      /home/linuxadmin/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.5.0 build exe:indent-puzzle-exe --ghc-options " -ddump-hi -ddump-to-file"

    Process exited with code: ExitFailure 1

[linuxadmin@trac-leeds-09-vm01 13:08:00 ~/indent-puzzle ]

$ stack ghci

indent-puzzle-0.1.0.0: build

Preprocessing executable 'indent-puzzle-exe' for indent-puzzle-0.1.0.0...

[1 of 1] Compiling Main             ( app/Main.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/indent-puzzle-exe/indent-puzzle-exe-tmp/Main.o )

/home/linuxadmin/indent-puzzle/app/Main.hs:5:10: Empty 'do' block

--  While building package indent-puzzle-0.1.0.0 using:

      /home/linuxadmin/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.5.0 build exe:indent-puzzle-exe --ghc-options " -ddump-hi -ddump-to-file"

    Process exited with code: ExitFailure 1

Warning: build failed, but optimistically launching GHCi anyway

The following GHC options are incompatible with GHCi and have not been passed to it: -threaded

Using main module: 1. Package `indent-puzzle' component exe:indent-puzzle-exe with main-is file: /home/linuxadmin/indent-puzzle/app/Main.hs

Configuring GHCi with the following packages: indent-puzzle

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help

[1 of 1] Compiling Main             ( /home/linuxadmin/indent-puzzle/app/Main.hs, interpreted )

Ok, modules loaded: Main.

*Main> main

A

*Main>

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