Comments about stricteness of catch#

In "Improve strictness analysis for exceptions"
   commit 7c0fff41789669450b02dc1db7f5d7babba5dee6
   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.

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