Good call. I see `orM` is actually a wrapper of the "ugly" nested-if. So operators can be another wrapper away. Thanks. On Mon, May 27, 2019 at 3:35 PM Emil Axelsson <78emil@gmail.com> wrote:

Not with the standard `||` and `&&` operations. You'd have to define new versions with a `Monad` constraint.

The `monad-loops` package is pretty nice for short-circuiting stuff. See, for example, `andM` and `orM`. But it doesn't provide any binary operators.

https://hackage.haskell.org/package/monad-loops

/ Emil

Den 2019-05-27 kl. 07:07, skrev Magicloud Magiclouds:

...

Hi,

I think `a || b && c` is more clear than ``` if a then True else if b then if c then True else False else False ```

And some languages, `pureComputing || (ioOperation && pure2)` involves IO only when pureComputing is False.

So in Haskell, is it possible to get both benefits? ``` status <- ioOperation -- IO-ed anyway return $ pureComputing || (status && pure2) ``` ``` if pureComputing then return True else do status <- ioOperation if status then return pure2 else return False -- Looks ugly ``` _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Cool. Thanks. On Mon, May 27, 2019 at 5:10 PM Viktor Dukhovni wrote:

...

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)

you'd write:

pure preComputing <||> (ioOperation <&&> pure pure2)

You could even define suitable fixity to make <&&> have higher precedence than <||> and not need any parentheses.

-- Viktor.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

With side effect, they are not the same. Viktor's nested-if does not trigger unnecessary IO action, while liftA2 does. On Mon, May 27, 2019 at 5:38 PM Jack Kelly wrote:

Viktor Dukhovni writes:

...

...

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 (||)

-- Jack _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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

Viktor Dukhovni writes:

...

...

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.