Re: [Haskell-cafe] Let it be

I don't know of any offhand that specifically call it out -- it's a natural consequence of the layout rule which is described in the Haskell Report. However, there is at least one ticket in Haskell' to fix it for if/then/else: http://hackage.haskell.org/trac/haskell-prime/ticket/23 -Ross On Oct 8, 2009, at 1:03 PM, michael rice wrote:

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.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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