Haskell and Tabs

Today, I learned Haskell doesn’t like tabs. Haskell apparently uses some syntax conventions to connote things like blocks. I was working my way through a Haskell tutorial when I found the following snippet of code kept getting compiler errors:

fails
main = do putStr "You are? "
          name <- getLine
          greet name

Turns out you can also enclose a block in angle brackets so this code worked

compiles
main = do { putStr "You are? ";
name <- getLine;
greet name }

Then it dawned on me. My TextMate Haskell setting were showing

instead of

with spaces. Flipping it to use spaces made the same code pass compilation.

compiles
main = do putStr "You are? "
          name <- getLine
          greet name

Experimenting with lining up the arguments after the do showed that all of the statements had to line up vertically. Thus this reasonable looking syntax would fail:

fails
main = do putStr  "You are? "
                  name <- getLine
                  greet name 

I’m still very much at the beginnings of digging into Haskell, but it’s nice to play around with a language purely for intellectual curiosity.