diff options
Diffstat (limited to 'compiler/prelude/primops.txt.pp')
| -rw-r--r-- | compiler/prelude/primops.txt.pp | 43 |
1 files changed, 28 insertions, 15 deletions
diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp index c29e9d825a..9b107f2053 100644 --- a/compiler/prelude/primops.txt.pp +++ b/compiler/prelude/primops.txt.pp @@ -1919,18 +1919,34 @@ primop CasMutVarOp "casMutVar#" GenPrimOp section "Exceptions" ------------------------------------------------------------------------ --- 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 +{- 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. +-} primop CatchOp "catch#" GenPrimOp (State# RealWorld -> (# State# RealWorld, a #) ) @@ -1938,9 +1954,6 @@ primop CatchOp "catch#" GenPrimOp -> State# RealWorld -> (# State# RealWorld, a #) with - -- 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. strictness = { \ _arity -> mkClosedStrictSig [strictApply1Dmd,lazyApply2Dmd,topDmd] topRes } -- See Note [Strictness for mask/unmask/catch] out_of_line = True |