Test.ChasingBottoms.TimeOut

When dealing with "hard bottoms", i.e. non-terminating computations that do not result in exceptions, the following functions may be handy.

Note that a computation is considered to have terminated when it has reached weak head normal form (i.e. something distinct from bottom).

Documentation

data Result a

Constructors

Value a

NonTermination

Exception SomeException

Instances

Show a => Show (Result a)

Defined in Test.ChasingBottoms.TimeOut

Methods

showsPrec :: Int -> Result a -> ShowS

show :: Result a -> String

showList :: [Result a] -> ShowS

timeOut :: Int -> IO a -> IO (Result a)

timeOut n c runs c for at most n seconds (modulo scheduling issues).

• If the computation terminates before that, then Value v is returned, where v is the resulting value. Note that this value may be equal to bottom, e.g. if c = return bottom.
• If the computation does not terminate, then NonTermination is returned.
• If the computation raises an exception, then Exception e is returned, where e is the exception.

Note that a user-defined exception is used to terminate the computation, so if c catches all exceptions, or blocks asynchronous exceptions, then timeOut may fail to function properly.

timeOut' :: Int -> a -> IO (Result a)

timeOut' is a variant which can be used for pure computations. The definition,

  timeOut' n = timeOut n . evaluate

ensures that timeOut' 1 bottom usually returns Exception <something>. (timeOut 1 (return bottom) usually returns Value bottom; in other words, the computation reaches whnf almost immediately, defeating the purpose of the time-out.)

timeOutMicro :: Int -> IO a -> IO (Result a)

timeOutMicro takes a delay in microseconds. Note that the resolution is not necessarily very high (the last time I checked it was 0.02 seconds when using the standard runtime system settings for GHC).

timeOutMicro' :: Int -> a -> IO (Result a)

timeOutMicro' is the equivalent variant of timeOutMicro:

 timeOutMicro' n = timeOutMicro n . evaluate