Duh! Of course, thanks David.

On 2 Dec 2020, at 12:51, David McBride wrote:

If you check on hoogle for how guard is written, it is just this guard True = pure https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.h... () guard False = empty https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.h... That means you can use the same thing in your own code

import Control.Applicative

pairs xs = xs >>= \x -> xs >>= \y -> if (x + y == 2020) then pure (x,y) else empty

On Wed, Dec 2, 2020 at 5:31 AM mike h mailto:mike_k_houghton@yahoo.co.uk> wrote: Hi, I have sumIs2020P1' xs = do x <- xs y <- xs guard (x + y == 2020) pure (x,y)

which has been desugared from a list comprehension I would like to reduce this even more using >>= So I do sumIs2020P1'' xs = (a,b) where (a,b):rest = filter (\(x,y) -> x + y == 2020) pairs

pairs = xs >>= \x -> xs >>= \y -> pure (x,y)

but really I would like the guard to be within the >>= sections but I could not work out how to do it! i.e. I’m looking for something like (pseudo code)

pairs = xs >>= \x -> xs >>= \y -> if (x + y == 2020) then pure (x,y) else DO_NOTHING

which would then allow the filter to be removed.

Many Thanks

Mike

Dr Mike Houghton

mike_k_houghton@yahoo.co.uk mailto:mike_k_houghton@yahoo.co.uk

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