Exception handling in GHC

[Sarah Thompson]

I've noticed some interesting behaviour:

Prelude Control.Exception> try (return (error "e"))
Prelude Control.Exception> it
Right *** Exception: e

It would appear that when the result of this function is evaluated, the
exception fires during evaluation, after the try is out of scope. I suppose
it makes some kind of twisted sense due to lazy evaluation going on, but it
is certainly a gotcha.

I did notice this:

Prelude Control.Exception> try (return $! (error "e"))
Prelude Control.Exception> it
Left e

Seemingly, forcing strict evaluation on the argument of return fixes the
problem by making sure that any exception that *can* happen, *does* happen.

However, this code from my COM wrapper:

--[id(3)] HRESULT XmlQuery([in,string] BSTR query, [out,retval]BSTR* xml);
xmlQuery :: String -> State -> IO String
xmlQuery qry (State st) = do
    db <- readIORef st
    Control.Exception.catch (return $! (XMLDatabase.xmlQuery qry db))
                            (\ _ -> return "<error description=\"Parse
error\"/>")

doesn't work. If XMLDatabase.xmlQuery throws an exception, the COM wrapper
still terminates the host application (Yes, Sigbjorn - I did apply your
patch to HDirect, but it seemed to make no difference).

I'm still a bit stuck here!

Sarah