Re: How to short-circuit match failure?

*To*: mathgroup at smc.vnet.net*Subject*: [mg114497] Re: How to short-circuit match failure?*From*: kj <no.email at please.post>*Date*: Sun, 5 Dec 2010 21:56:43 -0500 (EST)*References*: <ictfq8$mck$1@smc.vnet.net> <id01ld$h6o$1@smc.vnet.net> <id4sku$cge$1@smc.vnet.net>

In <id4sku$cge$1 at smc.vnet.net> kj <no.email at please.post> writes: >ClearAll[foo] >foo::toolong = "First argument is too long"; >foo::nolist = "First argument is not a list"; >foo::nargs = "foo called with `1` argument(s); 2 expected"; >foo[args___] := Module[{chkargs, argsok, core, othervars}, > > chkargs[x_List /; Length[x] < 3, y_] := True; > > chkargs[_List, _] := Message[foo::toolong]; > chkargs[_, _] := Message[foo::nolist]; > chkargs[x___] := Message[foo::nargs, Length[{x}]]; > > core[x_, y_] := {#, y} & /@ x; > > argsok = chkargs[args]; > argsok && core[args] /; argsok > ] Actually, there's an unexpected problem with this solution. As I reported in a different thread (Message-ID: <idd7ml$nae$1 at smc.vnet.net>, Subject: [mg114497] Tempvar zombies littering my context!), each evalution of foo[...] would lead to the addition of lingering "temporary" variables to the context. Therefore, if a function defined as proposed above was called many times (e.g. in a loop with many iterations), this would lead to an explosion in the number of variables that would have to be kept track of in the context. One workaround is to change Module to Block, since Block does not create any temporary variables in the first place. Of course, using Block is trickier than using Module, because there's the risk that a block variable will localize the value of a *global* variable that is used by some other function invoked during the evaluation of the Block. (There may be a way to use Unique in a way that automatically prevents this problem with Block without requiring any particular care from the programmer, but, if so, it is beyond my Mathematica skill to devise it.) ~kj