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MathGroup Archive 1990

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evaluation

  • To: mathgroup at yoda.ncsa.uiuc.edu
  • Subject: evaluation
  • From: uunet!cello.hpl.hp.com!jacobson
  • Date: Fri, 23 Mar 90 08:49:33 PST

Does anyone know how evaluation really works?  I submit this example
of funny behavior (compressed to single spacing for ease of reading):

In[1]:= foo[x_] := BAZZZ /; (Print["hi there"];y===True)
In[2]:= a = foo[xxx]
hi there
Out[2]= foo[xxx]
In[3]:= b=a
Out[3]= foo[xxx]
In[4]:= y=True
Out[4]= True
In[5]:= c=a
Out[5]= foo[xxx]
In[6]:= foo[xxx]
hi there
Out[6]= BAZZZ

Output 5, in particular, seems rather peculiar.  At that point y is True,
so it should be impossible to print anything that matches foo[_].

  -- David Jacobson




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