[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:

> I have a list comprehension that extracts the singletons from xs using f
> and g, and creates a pair from their output:
>
>         [(a,b) | a <- f xs, b <- g xs]
>
> I executed this and the result is the empty list:
>
>         []
>
> That is odd. Why is the empty list the result?
>

For the same reason that the cartesian product of any set with the empty
set is the empty set.

Also,

> I have a list of singletons:
>        xs = [("a")]

is immediately suspect, since you have xs:: [String], whereas "a list of
singletons" appears to lean quite a lot more toward [Char], which is just
String.

-- Kim-Ee
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