strlen in Hutton 2ed 10.5 Derived primitives White Spaced run on Windows 7 Haskell Platform 8.2.2 strlen :: IO () GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help strlen = do putStr "Enter a string: " [1 of 1] Compiling Main ( C:\strlen.WhiteSpaced.hs, interpreted ) xs <- getLine putStr "The string has " C:\strlen.WhiteSpaced.hs:2:14: error: putStr (show (length xs)) Parse error in pattern: putStr putStr " characters" Possibly caused by a missing 'do'? | 2 | strlen = do putStr "Enter a string: " | ^^^^^^^^^^^^^^^^^^^^^^^^^^^... Failed, no modules loaded. Prelude> Bracesed strlen :: IO GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help strlen = do {putStr "Enter a string: "; [1 of 1] Compiling Main ( C:\strlen.Bracesed.hs, interpreted ) xs <- getLine; Ok, one module loaded. putStr "The string has "; *Main> putStr (show (length xs)); putStr " characters"} -- Sent from: http://haskell.1045720.n5.nabble.com/Haskell-Haskell-Cafe-f3074699.html

Some ways to fix the given code: strlen :: IO () strlen = do  putStr "Enter a string: " >> do                   xs <-  getLine                   putStr "The string has "                   putStr (show (length xs))                   putStr " characters" strlen :: IO () strlen = do  putStr "Enter a string: "              do                   xs <-  getLine                   putStr "The string has "                   putStr (show (length xs))                   putStr " characters" strlen :: IO () strlen = do  putStr "Enter a string: "              xs <-  getLine              putStr "The string has "              putStr (show (length xs))              putStr " characters" On 15/05/2018 18:13, Sylvester via Haskell-Cafe wrote:

strlen in Hutton 2ed 10.5 Derived primitives => run on Windows 7 Haskell Platform 8.2.2 strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStr " characters" =================================================== GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( C:\strlen.WhiteSpaced.hs, interpreted )

C:\strlen.WhiteSpaced.hs:2:14: error: Parse error in pattern: putStr Possibly caused by a missing 'do'? | 2 | strlen = do putStr "Enter a string: " | ^^^^^^^^^^^^^^^^^^^^^^^^^^^... Failed, no modules loaded. Prelude> ================================================== strlen :: IO () strlen = do {putStr "Enter a string: "; xs <- getLine; putStr "The string has "; putStr (show (length xs)); putStr " characters"} ================================================= GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( C:\strlen.Bracesed.hs, interpreted ) Ok, one module loaded. *Main>

-- Sent from: http://haskell.1045720.n5.nabble.com/Haskell-Haskell-Cafe-f3074699.html _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Hi Victor, It is frustrating that posting plain text is contorted (via nabble.com and then again in my mail.google.com account) - please see the attachment. I really have sent the right indentation copping the Hutton's script. Is it so difficult for you guys to replicate this obvious failure of GHC on "do notation" with white space? Regards, Sylvester On Tue, May 15, 2018 at 6:27 PM, Viktor Dukhovni wrote:

...

On May 15, 2018, at 12:13 PM, Sylvester via Haskell-Cafe < haskell-cafe@haskell.org> wrote:

strlen in Hutton 2ed 10.5 Derived primitives => run on Windows 7 Haskell Platform 8.2.2 strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStr " characters" ===================================================

The above indentation looks wrong. Try:

---------- $ cat foo.hs strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStrLn " characters"

$ ghci foo.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( foo.hs, interpreted ) Ok, modules loaded: Main. Loaded GHCi configuration from /tmp/haskell-stack-ghci/ 05485125/ghci-script *Main> strlen Enter a string: foo The string has 3 characters *Main> ----------

If I incorrectly indent the subsequent lines:

strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStrLn " characters"

Then I see an error:

foo.hs:2:13: error: Parse error in pattern: putStr Possibly caused by a missing 'do'?

-- Viktor.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

It looks like you may be trying to do this with a proportional font, which is not going to work very well. You will have much better luck if you use a fixed width font when you need to line up columns where some of the lines have non-whitespace characters. An alternative approach would be to have a line break immediately after "do", so all of the lines that need to be indented have only preceding whitespace. The hindent code formatter uses this style, as you can see in any of its examples with do blocks, such as this: https://github.com/commercialhaskell/hindent/blob/master/TESTS.md#function-d... - when code is written in this style, the blocks will line up visually whether or not the font is fixed width. On Tue, May 15, 2018 at 10:02 AM Sylvester Hazel via Haskell-Cafe < haskell-cafe@haskell.org> wrote:

Hi Victor, It is frustrating that posting plain text is contorted (via nabble.com and then again in my mail.google.com account) - please see the attachment. I really have sent the right indentation copping the Hutton's script. Is it so difficult for you guys to replicate this obvious failure of GHC on "do notation" with white space? Regards, Sylvester

On Tue, May 15, 2018 at 6:27 PM, Viktor Dukhovni wrote:

...

...

On May 15, 2018, at 12:13 PM, Sylvester via Haskell-Cafe < haskell-cafe@haskell.org> wrote:

strlen in Hutton 2ed 10.5 Derived primitives => run on Windows 7 Haskell Platform 8.2.2 strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStr " characters" ===================================================

The above indentation looks wrong. Try:

---------- $ cat foo.hs strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStrLn " characters"

$ ghci foo.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( foo.hs, interpreted ) Ok, modules loaded: Main. Loaded GHCi configuration from /tmp/haskell-stack-ghci/05485125/ghci-script *Main> strlen Enter a string: foo The string has 3 characters *Main> ----------

If I incorrectly indent the subsequent lines:

strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStrLn " characters"

Then I see an error:

foo.hs:2:13: error: Parse error in pattern: putStr Possibly caused by a missing 'do'?

-- Viktor.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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On 2018-05-15 10:13 AM, Sylvester via Haskell-Cafe wrote:

strlen in Hutton 2ed 10.5 Derived primitives => run on Windows 7 Haskell Platform 8.2.2 strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStr " characters"

This works fine if you indent the code correctly. The code following the first putStr should line up with it vertically: strlen :: IO () strlen = do putStr "Enter a string: " xs <- getLine putStr "The string has " putStr (show (length xs)) putStr " characters" This compiles for me with ghc 8.2. The way you have the code above, the second and subsequent lines of the do block are indented further, and so would need an additional do. However, I don't think this is what's intended. Are you using tabs by any chance? Or have you copied the source from text that uses a mixture of tabs and spaces?