Primops for making exceptions precise or hiding continuation strictness in IO

Currently we consider raiseIO# to be the only source of precise exceptions. This is really limiting.

Here are some basic operations we might want to provide:

• considerExceptionsPrecise#
  • :: (State# RealWorld -> (# State# RealWorld, a_reppoly #)) -> State# RealWorld -> (# State# RealWorld, a_reppoly #)
  • considerExceptionsPrecise# a s is equivalent to a s except that when any synchronous exception (precise or imprecise) is raised by the the latter, it becomes a precise exception in the former.
  • With our current precise exceptions mechanism, this means that we must hide strictness from the continuation of considerExceptionsPrecise# a s unless it can be proven that a s does not raise a synchronous exception.
• hideContinuationStrictness#
  • :: (State# RealWorld -> (# State# RealWorld, a_reppoly #)) -> State# RealWorld -> (# State# RealWorld, a_reppoly #)
  • At runtime, hideContinuationStrictness# a s is equivalent to a s. But case hideContinuationStrictness# a s of (# s', r #) -> cont #) is lazy in cont. This is similar in effect to our precise exceptions mechanism, but is not tied to whether the action a s throws an exception.
  • Or perhaps we could instead use evalBarrier# :: State# RealWorld -> State# RealWorld and define hideContinuationStrictness# a s = case considerExceptionsPrecise# a s of (# s2, r #) -> case evalBarrier# s2 of s3 -> (# s3, r #). (The considerExceptionsPrecise# here is not optional: If a s diverges we will drop the evalBarrier# in the continuation. We can consider an exception to be precise iff it includes an eval barrier.)