On Fri, Jul 06, 2007 at 12:08:01PM -0700, Dan Weston wrote:
From Haskell' ticket #76: http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns
The main idea is to add a single new production to the syntax of patterns pat ::= !pat
Experiments (ghci -fbang-patterns -O2 -S, rename identifiers, then diff)
You could have just used -ddump-ds... Core is way more readable than GHC assembly.
shows that nested pattern bindings are equivalent to the outermost binding:
!(!pat) ==> !pat !(~pat) ==> !pat
~(~pat) ==> ~pat ~(!pat) ==> ~pat
but I do not see any wording to that effect either in the Haskell 98 report, the GHC documentation, or the Haskell' wiki. Have I overlooked it, or does it follow from the existing language definition?
Pattern matching is completely specified, and from the rules I can derive a counter-example to your assertion. stefan@stefans:~$ ghci -fbang-patterns -v0 Prelude> case 3 of !(!2) -> 'a' *** Exception: <interactive>:1:0-21: Non-exhaustive patterns in case Prelude> case 3 of !(~2) -> 'a' 'a' Prelude> stefan@stefans:~$ Stefan