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

On Tue, Jul 23, 2013 at 3:53 PM, Costello, Roger L. 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