allPairs2 can be simplified using a trick I wouldn't dare use in
any language but Haskell:
triangle4 xs = fused undefined [] xs
where fused x (y:ys) zs = (x,y) : fused x ys zs
fused _ [] (z:zs) = fused z zs zs
fused _ [] [] = []
I submit this just for grins; it happens to be a touch faster but
not enough to bother about.
While the result _isn't_ all possible pairs of elements, making
the allPairs name a bit dodgy, it _does_ have O(|xs|**2) of them...
I would be surprised if the relative performance of two
approaches in ghci were always the same as their relative
performance in ghc.
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