pattern matching quirks
- To: mathgroup at smc.vnet.net
- Subject: [mg29351] pattern matching quirks
- From: "Michael" <michael at science.edu>
- Date: Thu, 14 Jun 2001 02:27:27 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
I am trying to write a function which matches only on a specific pattern of arguments, some of which can be optional. The problem I am having is that certain complex patterns aren't being matched under conditions which should match. For example: In[1] := test[1, inputs : (X["4"] | X[X[_Integer]] ...) : X["4"]] := inputs In[2] := test[1] Out[2] := X[4] This is the expected behavior. However if I redefine the function so that the alternatives are collected within a single head, it doesn't work: In[3] := Clear[test]; test[1, inputs : (X["4" | X[_Integer] ...]) : X["4"]] := inputs In[4] := test[1] Out[4] := test[1] In[5] := MatchQ[X["4"], (X["4" | X[_Integer] ...])] Out[5] := True MatchQ says that my default values matches the given pattern. Through some experimentation I had discovered that the function will NOT be called if the default value does not match the pattern for the optional argument. This is annoying (and undocumented I think) because I would like to insert a function call to generate the default value. This does not work right either because the default value appears to be evaluated when the function is defined, and wrapping it with Hold causes it to fail the above pattern match. So instead I have to assign it to some inane value (hence the alternatives) and then do a rule transformation in the function itself (not shown above). So here are my questions: 1. Why does the default value for a pattern have to match that pattern? 2. Why is the default value seeming to be evaluated when the function is defined? 3. Why do some patterns pass MatchQ but not result in a function call? Notably, X[a|b] versus X[a]|X[b]? 4. What other quirks should I be aware of that I haven't run into yet? :-) Regards, Michael Schmidt michael at science.edu