I'm looking at Bird's Thinking Functionally with Haskell and he gives two list comprehensions, asking under what conditions they deliver the same results
[e | x <- xs, p x, y <- ys]
[e | x <- xs, y <- ys, p x]
First, I'm confused about what is the input and what is the predicate. The y <- ys in the first LC seems to be in a predicate position, and in the second it's a second input after x <- xs with p x in the predicate position . . . confusing me.
The answer examples Bird gives are beyond me:
They deliver the same result only if ys is a finite list:
> [1 | x <- [1,3], even x, y <- undefined]
[]
> [1 | x <- [1,3], y <- undefined, even x]
Exception: Prelude.undefined
> [1 | x <- [1,3], y <- [1..], even x]
{Interruped}
I'm not sure what's being said here, or what points are being made.