conditional pattern question
- To: mathgroup at smc.vnet.net
- Subject: [mg15687] conditional pattern question
- From: dreissBLOOP at bloop.earthlink.net (David Reiss)
- Date: Sun, 31 Jan 1999 03:06:09 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
I don't understand the following behavior: could someone educate me
here? ...or is it a bug?
First clear the functions and variables.
In[1]:=
ClearAll[NumericWithMessageQ,f];
Now define a message:
In[2]:=
Test::testmessage="The argument `1` has generated this message.";
Introduce a boolian test that has the added side effect of generating a
message when the test yields False (I am using NumericQ as an example,
but the problem that I am illustrating is independent of this):
In[3]:=
NumericWithMessageQ[z_]:=
If[NumericQ[z],
True,
Message[Test::testmessage,z];False]
Now give a function definition for numeric values of the argument
In[4]:=
f[y_?NumericWithMessageQ]:=1;
Also give a definition for all other cases: the message defind a bove is
generated when this is executed-- I didn't expect this to happen. That
the message is generated at this point puzzels me. I don't understand
why this happens.
In[5]:=
f[y_]:=2;
Test::"testmessage": "The argument \!\(y_\) has generated this message."
Note though that the function works properly. Here is a numeric case:
In[6]:=
f[4]
Out[6]=
1
And here is a non-numeric case (and the message is generated as per the
design).
In[7]:=
f[a]
Test::"testmessage": "The argument \!\(a\) has generated this message."
Out[7]=
2
If the function is specified as follows, the message is not generated
unexpectedly when the function is defined.
In[8]:=
ClearAll[f]
In[9]:=
f[y_]/;NumericWithMessageQ[y]:=1;
In[10]:=
f[y_]:=2;
And the function works as above.
In[11]:=
f[4]
Out[11]=
1
In[12]:=
f[a]
Test::"testmessage": "The argument \!\(a\) has generated this message."
Out[12]=
2
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