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