Hi, Dnia 2013-07-23, wto o godzinie 08:53 +0000, Costello, Roger L. pisze:
Hi Folks,
I have a list of singletons:
xs = [("a")]
f is a function that, given an argument x, it returns the argument:
f x = x
g is a function that, given an argument x, it returns the empty list:
g x = []
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?
Becouse g xs has zero elements. Look at this example: [(x,x) | x <- []] => [] Empty list has zero elements.
Consider this list comprehension that extracts the singletons from xs using f only, and creates a pair by matching up the singleton from xs with the empty list:
[(a,[]) | a <- f xs]
The result is this:
[("a",[])]
That is the result that I expected to get with the first list comprehension.
Would you explain why this:
[(a,b) | a <- f xs, b <- g xs]
yields [], whereas this:
[(a,[]) | a <- f xs]
yields [("a",[])]?
Also, how would I modify this:
[(a,b) | a <- f xs, b <- g xs]
so that it produces this:
[("a",[])]
Define g as: g xs = [[]] or maybe better: g _ = [[]] Now it has 1 element - empty list. Best regards, Emanuel