Re: Pattern test and expression sequence

*To*: mathgroup at smc.vnet.net*Subject*: [mg83143] Re: Pattern test and expression sequence*From*: Szabolcs Horvát <szhorvat at gmail.com>*Date*: Tue, 13 Nov 2007 03:33:17 -0500 (EST)*References*: <fh9903$ap6$1@smc.vnet.net>

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"). This is because x__?myQ only matches if myQ gives True for each argument separately, i.e.\ in this case f[1,2] evaluates only if both myQ[1] and myQ[2] are True. > 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. > Why do you find it uglier than using '?' ? You could simply write f[x__] /; myQ[x] := works[x] or f[x__] := works[x] /; myQ[x] Szabolcs