Re: Pattern test and expression sequence

*To*: mathgroup at smc.vnet.net*Subject*: [mg83144] Re: Pattern test and expression sequence*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Tue, 13 Nov 2007 03:33:48 -0500 (EST)*Organization*: Uni Leipzig*References*: <fh9903$ap6$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de

Hi, x__?myQ is a pattern that is satifyed by a single x with myQ[x]===True if the sequence of x__ has more than a single element than *all* elements must satisfy the test myQ[] and since you test fail for a single element the tests for the other elements are not done anymore. Regards Jens Jan Lellmann wrote: > Hello, > > I want to define a pattern that accepts a sequence of expressions (of > arbitrary length) that matches some condition. So as a minimalist > example, if I wanted to accept all expressions with f as head and at > least 2 arguments, I want to write > > myQ[x__] := (Print[x]; Length[{x}] > 1) > f[x__?myQ] := works[x]; > f[1, 2] > > This should print "1 2" and return "works[1,2]". But it doesn't, it just > prints "1" and returns f[1,2] unevaluated. So apparenty Mathematica does > not pass the complete expression sequence to myQ, but only the first > element ("1"). > > There is a workaround using /; : > f[x__] /; (myQ[x] == True) := works[x]; > > but it is rather ugly and cumbersome when there is more than one argument. > > Does anyone have an idea why Mathematica does this? Is it a bug? Or is > there a (nice) workaround? > > Best regards, > Jan >