Re: [Haskell-cafe] Let it be

8 Oct 2009

      You're running into this problem because you're in a do-block. In a do- 
block, all the continuing lines of a single statement in the do-block  
must be indented w/r/t to the first line. The cylinder example doesn't  
have this issue because it's not in a do-block.

The layout rule (I'm summarizing and I haven't read the spec, so  
someone jump on me if I'm stating the wrong thing) basically says that  
within some implied { }, ;'s will be inserted for any line that is at  
the same indentation level as the previous line, e.g.

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

has the following implied symbols

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

which is now obviously wrong, as the let and in are in two separate  
statements.

Conversely,

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

implied:

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

Hope that clarifies things.

-Ross

On Oct 8, 2009, at 11:43 AM, michael rice wrote:
...
From Learn You a Haskell ("Let it be" section):
1. cylinder :: (RealFloat a) => a -> a -> a
   2. cylinder r h =
   3.     let sideArea = 2 * pi * r * h
   4.         topArea = pi * r ^2
   5.     in  sideArea + 2 * topArea
===================
What's the proper indentation for LET so these problems (below)  
don't arise? I thought LET and IN should be aligned in the same  
column. Also, isn't a LET expression an "expression."
Michael
==============
This works:
import System.Random
main = do
  gen <- getStdGen
  let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen)
    in putStrLn $ "Number is " ++ show randNumber
==============
This works:
import System.Random
main = do
  gen <- getStdGen
  let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen)
  putStrLn $ "Number is " ++ show randNumber
==============
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
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Re: [Haskell-cafe] Let it be

Ross Mellgren