> 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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