Right. Still, it's a point worth noting. I've added a short section to http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns which in turn is referred to from the GHC manual. Simon From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Lennart Augustsson Sent: 18 November 2007 22:47 To: Derek Elkins Cc: Haskell users Subject: Re: [Haskell] Bang patterns and declaration order I totally agree with Derek. Which exception you get can vary with compiler version, compiler flags, time of day, phase of the moon, ... It will be one in a set of exceptions, but you don't know which one. -- Lennart On Nov 18, 2007 8:34 PM, Derek Elkins <derek.a.elkins@gmail.com<mailto:derek.a.elkins@gmail.com>> wrote: On Sun, 2007-11-18 at 12:11 -0800, Iavor Diatchki wrote:
Hello,
I was playing around with "bang patterns" and I noticed that when combined with asynchronous exceptions they can lead to programs where the order of the declarations in a binding group is important! Here is an example:
import Control.Exception import Prelude hiding (catch)
main = putStrLn =<< eval_order
test = "no exception" where !_ = error "top down" !_ = error "bottom up"
eval_order = evaluate test `catch` \e -> case e of ErrorCall txt -> return txt _ -> throw e
Of course, this is a contrived exampled but, as far as I know, this was not possible in Haskell before (if anyone has an example to the contrary please send it to the list).
By the way, with GHC 6.8.1 the above program prints "bottom up". This means that when there are multiple "banged" bindings they are evaluated starting with the last one in the text. I imagine than in most programs this is not particularly important, but it seems to me that it would be a bit nicer if we were to adjust the translation so that bindings were evaluated top to bottom (e.g., like in ML). The whole point of the "imprecise exceptions" paper was that any exception may be returned when multiple ones could be. There is no reason why the bindings should be evaluated top-down. If you are relying on the order the bindings are evaluated you are doing something very, very wrong. Should we also specify what exception should be thrown for error "left-right" + error "right-left" ?
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