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 <allb...@ece.cmu.edu> wrote:
From: Brandon S. Allbery KF8NH <allb...@ece.cmu.edu>
Subject: Re: [Haskell-cafe] Let it be
To: "michael rice" <nowg...@yahoo.com>
Cc: "Brandon S. Allbery KF8NH" <allb...@ece.cmu.edu>, 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
[mich...@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]
allb...@kf8nh.comsystem administrator [openafs,heimdal,too many hats]
allb...@ece.cmu.eduelectrical and computer engineering, carnegie mellon
university KF8NH
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