Let it be

newer
Generalizing IO

older
Test.QuickCheck: generate

michael rice

8 Oct 2009 8 Oct '09

11:43 a.m.

...

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

Attachments:

attachment.html (text/html — 6.6 KB)

Show replies by date

Ross Mellgren

8 Oct 8 Oct

11:51 a.m.

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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Brandon S. Allbery KF8NH

11:53 a.m.

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

Brandon S. Allbery KF8NH

12:01 p.m.

On Oct 8, 2009, at 11:53 , Brandon S. Allbery KF8NH wrote:

...

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.

I should probably clarify that what is actually inserted is a semicolon as noted by Ross Mellgren, and the next level of translation then replaces it with the (>>) operator.

...

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

I should also mention that there are still constructs where this indentation quirk can bite you, notably:

...

if foo then bar else baz

works outside of a "do" construct, but inside one you need extra spaces:

...

do if foo then bar else baz

otherwise, you again get semicolons inserted before the "then" and "else" and subsequently replaced with (>>). -- 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

michael rice

1:03 p.m.

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

Ross Mellgren

1:07 p.m.

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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Felipe Lessa

1:18 p.m.

On Thu, Oct 8, 2009 at 2:07 PM, Ross Mellgren wrote:

...

there is at least one ticket in Haskell' to fix it for if/then/else

...and there isn't one for let/in because you can use just let (without in) inside a do-block. Of course the meanings are different as in the first case the let-bound variables scope only the 'in ...' part, while without in it scopes the rest of the do block: do {ini; let ... in ...; rest} => ini >> (let ... in ...) >> rest do {ini; let ...; ...; rest} => ini >> (let ... in (... >> rest)) HTH, -- Felipe.

5706

Age (days ago)

5706

Last active (days ago)

List overview

Download

6 comments

4 participants

participants (4)

Brandon S. Allbery KF8NH
Felipe Lessa
michael rice
Ross Mellgren

Let it be

michael rice

Ross Mellgren

Brandon S. Allbery KF8NH

Brandon S. Allbery KF8NH

michael rice

Ross Mellgren

Felipe Lessa

tags

participants (4)