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Simon Peyton Jones authored
In "Improve strictness analysis for exceptions" commit 7c0fff41 Author: Simon Peyton Jones <simonpj@microsoft.com> Date: Tue Jul 21 12:28:42 2015 +0100 I made catch# strict in its first argument. But today I found a very old comment suggesting the opposite. I disagree with the old comment, but I've elaborated the Note, which I reproduce here: {- Note [Strictness for mask/unmask/catch] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this example, which comes from GHC.IO.Handle.Internals: wantReadableHandle3 f ma b st = case ... of DEFAULT -> case ma of MVar a -> ... 0# -> maskAsynchExceptions# (\st -> case ma of MVar a -> ...) The outer case just decides whether to mask exceptions, but we don't want thereby to hide the strictness in 'ma'! Hence the use of strictApply1Dmd. For catch, we know that the first branch will be evaluated, but not necessarily the second. Hence strictApply1Dmd and lazyApply1Dmd Howver, consider catch# (\st -> case x of ...) (..handler..) st We'll see that the entire thing is strict in 'x', so 'x' may be evaluated before the catch#. So fi evaluting 'x' causes a divide-by-zero exception, it won't be caught. This seems acceptable: - x might be evaluated somewhere else outside the catch# anyway - It's an imprecise eception anyway. Synchronous exceptions (in the IO monad) will never move in this way. There was originally a comment "Catch is actually strict in its first argument but we don't want to tell the strictness analyser about that, so that exceptions stay inside it." but tracing it back through the commit logs did not give any rationale. And making catch# lazy has performance costs for everyone.
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