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Question about Check[]

  • To: mathgroup
  • Subject: Question about Check[]
  • From: sdoyle at
  • Date: Tue, 2 May 89 09:46:39 CDT

[This is the first of a series of messages between Sean and me
which basically discuss some differences between version 1.1 of
Mathematica and a 1.2 beta release I am testing.   I do not know
the release date of 1.2 non-beta. -smc]

	I'm having some trouble getting Check[] to work with messages that
I've defined. This would make error trapping much easier! When I enter
the example from the manual (page 261), things work fine. Below is a 
similar example with a user-defined Message that doesn't work. 
	This might be something I've missed in the manual, although
examples of messages (such as for Convert on page 539) don't work
with Check[] either.

	The desired result would be for the symbol `y' in Out[8]
to have the value meaningless instead of `Null'.

	I've also tried using On[test::tosmall] without any difference
in results.

	So... how can I create messages that Check[] will trap? 


 Sean Doyle    
 Board of Governors of the Federal Reserve System 
 20 and Constitution, NW  Mail Stop 76  Washington, DC 20551
 (202) 452-2352     uucp: uunet!fed!m1swd00 , internet: sdoyle at

********************* Session below ************************
mqws2(115)% math
SunMathematica (sun3.68881) 1.1 (September 14, 1988) [With pre-loaded data]
by S. Wolfram, D. Grayson, R. Maeder, H. Cejtin,
   S. Omohundro, D. Ballman and J. Keiper
Copyright 1988 Wolfram Research Inc.
-- SunView graphics initialized --

In[1]:= test[x_]:= x /; x>0

In[2]:= test::toosmall = "The input to test is too small"

Out[2]= The input to test is too small

In[3]:= test[x_] := Message[test::toosmall]

In[4]:= ??test
test/: test[x_] := x /; x > 0
test/: test[x_] := Message[test::toosmall]

In[4]:= Messages[test]

Out[4]= test::toosmall -> The input to test is too small

In[5]:= test[5]

Out[5]= 5

In[6]:= test[-4]

test::toosmall: The input to test is too small

In[7]:= y = Check[test[7],meaningless]

Out[7]= 7

In[8]:= y = Check[test[-7],meaningless]

test::toosmall: The input to test is too small

In[9]:= y

In[10]:= FullForm[y]

Out[10]//FullForm= Null


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