On Oct 8, 2009, at 11:43 , michael rice wrote:

This doesn't:

import System.Random
main = do
gen <- getStdGen
let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen)
in putStrLn $ "Number is " ++ show randNumber

[michael@localhost ~]$ runhaskell zz.hs

zz.hs:4:2:
The last statement in a 'do' construct must be an expression

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.

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

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