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 <ietf-dane@dukhovni.org> 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.

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