Timing out pure functions
haskell, purely-functional, timeout
Solution
The point is that
return (fact 999999999)
immediately returns and doesn't trigger the timeout. It returns a thunk that will be evaluated later.
If you force evaluation of the return value,
main = do maybeNum <- timeout (10 ^ 7) $ return $! fact 99999999
print maybeNum
it should trigger the timeout (if you provide a stack large enough so that the timeout happens before the stack overflow).
Problem
How can I "kill" a pure calculation which is taking too long? I tried ``` import System.Timeout fact 0 = 1 fact n = n * (fact $ n - 1) main = do maybeNum <- timeout (10 ^ 7) $ (return . fact) 99999999 print maybeNum ``` However, this doesn't work. Replace the `(return . fact) 99999999` with a "real" IO function like `getLine` and this works as expected.