[Haskell-beginners] Desugar list comprehension

mike h mike_k_houghton at yahoo.co.uk
Wed Dec 2 10:30:37 UTC 2020

 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


Dr Mike Houghton

mike_k_houghton at yahoo.co.uk

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