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
[mich...@localhost ~]$ runhaskell zz.hs
zz.hs:4:2:
The last statement in a 'do' construct must be an expression
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