[Haskell-cafe] GHC TV station?

[Viktor Dukhovni]

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