Re: [Haskell-cafe] How to make boolean logic with IO Monad more expressive?

On Mon, May 27, 2019 at 07:37:37PM +1000, Jack Kelly wrote:

Viktor Dukhovni writes:

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On May 27, 2019, at 1:07 AM, Magicloud Magiclouds wrote:

``` status <- ioOperation -- IO-ed anyway return $ pureComputing || (status && pure2) ```

With two helpers:

(<&&>) :: IO Bool -> IO Bool -> IO Bool (<&&>) ma mb = ma >>= (\a -> if not a then return False else mb)

(<||>) :: IO Bool -> IO Bool -> IO Bool (<||>) ma mb = ma >>= (\a -> if a then return True else mb)

These generalise to any Applicative, I think:

import Control.Applicative (liftA2)

(<&&>) = liftA2 (&&) (<||>) = liftA2 (||)

As written, the above Applicative version won't short-circuit (is strict in both arguments) in the IO Monad. For example, the below will still read stdin: pure False <&&> (getLine >>= return . read) The generalized Monadic version will short-circuit. (<&&>) :: Monad m => m Bool -> m Bool -> m Bool (<&&>) ma mb = ma >>= (\a -> if not a then return False else mb) (<||>) :: Monad m => m Bool -> m Bool -> m Bool (<||>) ma mb = ma >>= (\a -> if a then return True else mb) -- Viktor.