On Oct 8, 2009, at 11:53 , Brandon S. Allbery KF8NH wrote:
The problem here is that the "do" construct parses things a bit differently; if you're at the same indentation level, it inserts a (>>) on the assumption that the next line is an independent expression, so you need to indent the "in" a bit more to avoid it.
I should probably clarify that what is actually inserted is a semicolon as noted by Ross Mellgren, and the next level of translation then replaces it with the (>>) operator.
Note however that this usage is common enough that "do" provides a shorthand: you can simply omit the "in", and "let" will be parsed as if it were a statement:
main = do gen <- getStdGen let (randNumber, newGen) = randomR (1,6) gen :: (Int,StdGen) putStrLn $ "Number is " ++ show randNumber
I should also mention that there are still constructs where this indentation quirk can bite you, notably:
if foo then bar else baz
works outside of a "do" construct, but inside one you need extra spaces:
do if foo then bar else baz
otherwise, you again get semicolons inserted before the "then" and "else" and subsequently replaced with (>>). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH