Thanks all,
So, in a do expression
let x = 1
y = 2
etc.
in z = 1 + 2
if <bool expr>
then
etc.
else
etc.
Is this deviation documented somewhere?
Michael
--- On Thu, 10/8/09, Brandon S. Allbery KF8NH wrote:
From: Brandon S. Allbery KF8NH
Subject: Re: [Haskell-cafe] Let it be
To: "michael rice"
Cc: "Brandon S. Allbery KF8NH" , haskell-cafe@haskell.org
Date: Thursday, October 8, 2009, 11:53 AM
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.comsystem administrator [openafs,heimdal,too many hats] allbery@ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH
