[Haskell-cafe] Let it be

michael rice nowgate at yahoo.com
Thu Oct 8 13:03:33 EDT 2009

Thanks all,

So, in a do expression 

 let x = 1
     y = 2
   in z = 1 + 2

  if <bool expr>

Is this deviation documented somewhere?


--- On Thu, 10/8/09, Brandon S. Allbery KF8NH <allbery at ece.cmu.edu> wrote:

From: Brandon S. Allbery KF8NH <allbery at ece.cmu.edu>
Subject: Re: [Haskell-cafe] Let it be
To: "michael rice" <nowgate at yahoo.com>
Cc: "Brandon S. Allbery KF8NH" <allbery at ece.cmu.edu>, haskell-cafe at 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 at localhost ~]$ runhaskell zz.hs

    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 at kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery at ece.cmu.eduelectrical and computer engineering, carnegie mellon university    KF8NH

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