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pattern bugs and comment on intuitive syntax for the New Year

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115143] pattern bugs and comment on intuitive syntax for the New Year
  • From: Richard Fateman <fateman at cs.berkeley.edu>
  • Date: Sun, 2 Jan 2011 04:56:08 -0500 (EST)


  Guess the results.

x+3 /. x+c___ -> aha[c]
x /. x+c___ -> aha[c]
x /. x+c___:0 -> aha[c]

compare to

x+y+3 /. x+y+c___ -> aha[c]
x+y  /. x+y+c___ -> aha[c]
x+y  /. x+y+c___:0 -> aha[c]
x+y+a+b  /. x+y+c___ :> aha[c]
x+y+a+b  /. x+y+c___ :> aha[Plus[c]]
x+y  /. x+y+c___ :> aha[Plus[c]]



I think the requirement for the :0 represents a bug. Maybe the
need for the Plus[], too. The treatment of NullSequences within
a Flat operator could be different, leaving an operator around.

............

Another item. really, syntax.

define

GreaterThan[q_]:= #>q&
gt2=GreaterThan[2]   (* a function of one argument.
                         is that argument > 2? *)


mm[2, 10] /. mm[q_, s_?GreaterThan[q]] :> aha[q, s]

  doesn't give aha..


mm[2, 10] /. mm[q_, s_?gt2] :> aha[q, s]

does give aha

this works, though.
mm[2, 10] /. mm[q_, s_?(GreaterThan[q])] :> aha[q, s]

I love that intuitive syntax. None of that excess
parentheses and stuff. (sarcasm)

This error would have been
much more apparent in Lisp.

happy new year.





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