why does the following code not cause an error? Instead, it exits immediately.

someAction :: IO (Either String ())
someAction = return $ Left "Error"

main :: IO ()
main = either error id <$> someAction

CodePudding user response：

When an action of type IO () is executed, its return value (the unit) is not evaluated. error, unlike ioError, is presenting as a pure function of type String -> a. To simplify your example:

main :: IO ()
main = pure $ error "error"

Running main as an IO action performs all side-effects (of which there are none) but doesn't do anything with the return value, so it is not evaluated.

This, however, will error, as print evaluates its argument (even ()):

main >>= print