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