NondecreasingIndentation is useful to model the imperative pattern // exit early if cond if (cond) return res; // otherwise continue bla in Haskell: if cond then return res else do bla If you have several exit conditions and have to indent every time you get ugly "staircase" code. However, it feels unintuitive that "then" and "else" do not set the indentation when they are first on a new line. Thus, you get valid Haskell which defies any aethestics: test = do if False then putStrLn "No" else case True of False -> putStrLn "No" True -> putStrLn "Yes" To be fair, Haskell is one of the first bigger languages with layout-sensitive parsing, and it is not surprising that the design is not smooth. On 15.11.2016 17:21, Bernhard Herzog wrote:

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 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#co...). With the option -XNoNondecreasingIndentation (or -XHaskell2010) GHC does produce an error message because of the empty do-block.

Bernhard _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel@gu.se http://www2.tcs.ifi.lmu.de/~abel/