[mg89225] Re: [mg89201] Bug in Default[] or Bug in My Understanding?

[Andrzej Kozlowski <akoz at mimuw.edu.pl>]

On 29 May 2008, at 20:03, Salvatore Mangano wrote:

> In[208]:= ClearAll[f] ;
> Default[f,2] = 0;
>
> In[210]:= f[a_, b_.] := a + b
>
> In[211]:= f[1]
> Out[211]= 1
>
> In[212]:= (* Now change Default *)
> Default[f,2] = 1;
>
> (* Note that Mathematica seems to recognize change *)
> In[213]:= ?f
> Global`f
> f[a_,b_.]:=a+b
>
> f/:Default[f,2]=1
>
>
> (* But not really! Expected 2 *)
> In[214]:= f[1]
> Out[214]= 1
>
> Even if it is a rule that you can't change the Default after the  
> function is defined it is certainly not nice of Mathematica to allow  
> such a change and advertise the change in the output of Definition[]  
> and Information[].
>
> I am using 6.0.2.0 on Windows XP.
>

I agree that this is confusing, but note that the documentation states  
pretty clearly:

The necessary values for  Default[f] must always be defined before  _.  
is used as an argument of  f.


I think the implication is quite clear: if you change Default you will  
have to evaluate the defintion of f again. Note that this works as  
expected:

In[1]:= ClearAll[f];
In[2]:= Default[f, 2] = 0;
In[3]:= f[a_, b_.] := a + b
In[4]:= f[1]
Out[4]= 1
In[5]:= Default[f, 2] = 1;
In[6]:= f[a_, b_.] := a + b
In[7]:= f[1]
Out[7]= 2

Andrzej Kozlowski