Am Sonntag 17 Januar 2010 11:05:47 schrieb Andrew Coppin:
Is there a specific reason why GHC consistently refuses to accept the following perfectly reasonable code snippet?
Yes, you violated the layout rule.
main = do putStrLn "Line 1" putStrLn "Line 2"
let xs = do x <- [1..10] y <- [1..10] return (x+y)
print xs
No matter which way I rearrange this, it *insists* that there's a parse error. This is very frustrating, given that it's utterly clear what I want...
It's not. ACLayout.hs:7:11: Empty 'do' construct should give a hint (line 7 is " let xs = do"). The next line after that is indented less than the "xs", so it ends the binding for xs (in fact, the entire let binding group) . You have to indent the lines in the do-block defining xs more than xs itself.