Today's code
-- haskell/read
main :: IO ()
main = do
let xs = [1, 2, undefined, 4] :: [Int]
print (take 2 xs)
What does this print? Why doesn't undefined crash it?
Answer
[1,2]. Haskell is lazy: take 2 xs only forces the first two cons cells of the list. The undefined sitting at index 2 is never touched, so it's never evaluated — and never throws.