[Haskell-beginners] Use of interact

[Francesco Ariis]

Ciao Michele,

On Sun, Feb 10, 2019 at 05:10:13PM +0100, Michele Alzetta wrote:
> If I leave my hello_worlds function as is and change the main function as
> follows:
> 
> main = interact $ show . hello_worlds . read::Int
> 
> I get: [...]
> 
> helloworlds.hs:14:8-44: error:
>     • Couldn't match expected type ‘Int’ with actual type ‘IO ()’
>     • In the expression: interact $ show . hello_worlds . read :: Int
>       In an equation for ‘main’:
>           main = interact $ show . hello_worlds . read :: Int

Two facts:

    - (.) is an operator which concatenates function
    - to concatenate functions, input/outputs must match

So let's analyse this:

    1. `read` has type `Read a => String -> a`
    2. `hello_worlds` has type `Int -> IO ()`
    3. `show` has type `Show a => a -> String`

and there is no way to convert `IO ()` to `String`. Remember that
hello_worlds does *not* return a series of Strings, but an IO action
(in this case, "blit something to screen")

Your `interact` example would function if written like this:

    main = interact $ unlines . map (hello_pure . read) . lines
    -- with `hello_pure :: Int -> String`

`lines` and `unlines` are there to keep input lazy for each line.
Do you think you can you fill-in "hello_pure" yourself?
-F