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