Re: unsafePerformIO to give warnings
On Fri, Dec 21, 2001 at 10:21:06AM -, Simon Marlow wrote: If I want to give warnings when doing something and I don't care too much about the order they appear in, can I use this? foo x = if success x then Just x else warn Working out x went wrong Nothing warn :: String - a - a warn s x = unsafePerformIO (hPutStrLn stderr s) `seq` x This is what IOExts.trace is for, BTW. Doh - I associate that with debugging and it never even occurred to me! warnings interspersed? It is possible to get warnings interspersed if you write something like warn (abc ++ warn foo def) Ah, yes, OK, thanks. hugs and ghci only seem to print the first warning, but ISTR similar problems happen with threads and got the impression that they were in the wrong. Are you sure that laziness and/or optimisation aren't to blame for only getting a single warning? Given that you say Hugs also produces the same result, it sounds like a laziness issue. Sorry, I meant hugs and ghci only seem to print the warning the first time I run the program, e.g. with module Main where import IOExts main :: IO() main = trace foo (putStrLn bar) in hugs: [...] Type :? for help Main main foobar Main main bar Main main bar Main main bar and in ghci: Skipping Main ( q.lhs, ./q.o ) Main main foo bar Main main bar Main main bar Main main bar Main Thanks again Ian ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: unsafePerformIO to give warnings
Ian Lynagh writes: Sorry, I meant hugs and ghci only seem to print the warning the first time I run the program, e.g. with module Main where import IOExts main :: IO() main = trace foo (putStrLn bar) in hugs: [...] Type :? for help Main main foobar Main main bar [snip] and in ghci: Skipping Main ( q.lhs, ./q.o ) Main main foo bar Main main bar [snip] This is because 'main' is a CAF: the act of reducing it to normal form the first time forces the warning to be printed, but thereafter the evaluated version is returned immediately. To subvert this behaviour, in GHCi you can say ':set +r' which causes all CAFs to be reverted between evaluations. Cheers, Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
unsafePerformIO to give warnings
Hi all If I want to give warnings when doing something and I don't care too much about the order they appear in, can I use this? foo x = if success x then Just x else warn Working out x went wrong Nothing warn :: String - a - a warn s x = unsafePerformIO (hPutStrLn stderr s) `seq` x The hslibs docs say If the I/O computation wrapped in unsafePerformIO performs side effects, then the relative order in which those side effects take place (relative to the main I/O trunk, or other calls to unsafePerformIO) is indeterminate. but it's not entirely clear on whether or not I could end up with 2 warnings interspersed? And is it guaranteed that the warnings will be printed at some point? hugs and ghci only seem to print the first warning, but ISTR similar problems happen with threads and got the impression that they were in the wrong. Thanks Ian ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
