As a concrete example, there's a nice function in Haskell's standard library called `interact :: (String -> String) -> IO ()`: https://hackage.haskell.org/packa

As a concrete example, there's a nice function in Haskell's standard library called `interact :: (String -> String) -> IO ()`: https://hackage.haskell.org/package/base-4.14.1.0/docs/Prelude.html#v:interact https://hackage.haskell.org/package/base-4.14.1.0/docs/Prelu... Its argument is a function of type `String -> String` and it returns an `IO ()`, i.e. an i/o action with a trivial result. That action will call the given function on contents of stdin, and writes its result to stdout. Or, equivalently, we can think of `interact f` as transforming a pure string-processing function `f` into a stdio CLI. Note that laziness (specifically "lazy IO") causes stdin to be read 'on demand', giving us a streaming computation without any extra work. Here's an example implementation of 'wc': module Main where import System.IO main :: IO () main = interact count count :: String -> String count input = show (length (unwords input)) Bonus: if we want to show off, we could implement 'count' using function composition like this: count = show . length . unwords