[Haskell-beginners] Why does this list comprehension return an empty list?

[Kim-Ee Yeoh]

On Tue, Jul 23, 2013 at 3:53 PM, Costello, Roger L. <costello at mitre.org>wrote:

> Also, how would I modify this:
>
>         [(a,b) | a <- f xs, b <- g xs]
>
> so that it produces this:
>
>         [("a",[])]
>

Try:  [(a,b) | a <- f xs | b <- g xs]

Note the 2nd parallel bar. You may have to enable a Parallel List
Comprehension pragma or somesuch.

Everyone else has given great explanations about your curious definitions
of f and g and why the original does what it does. Well worth looking into.

-- Kim-Ee
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