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MathGroup Archive 1999

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Re: Pure Functions in rules

  • To: mathgroup at
  • Subject: [mg16003] Re: Pure Functions in rules
  • From: tburton at (Tom Burton)
  • Date: Fri, 19 Feb 1999 03:27:14 -0500
  • Organization: Brahea, Inc.
  • References: <7ag34l$>
  • Sender: owner-wri-mathgroup at


Suggest using delayed evaluation (:>) of rule's RHS when that RHS
depends on a symbol (m here) introduced on the LHS.

Don't blame pure functions; blame Map. Any function mapped onto an
undefined symbol returns that symbol. When you use "->" instead of ":>".
you allow the function to operate before m is defined. That's why you
get "{1,2,3}". 


In[208]:= {1, 2, 3} /. m_List :> (2*#1 & ) /@ m
Out[208]= {2, 4, 6}

On 17 Feb 1999 22:58:13 -0500, in comp.soft-sys.math.mathematica you

>It appears that I cannot depend on using a pure function
>in a pattern-matching rule.
>In[79]:=     {1,2,3}/.m_List->f[m]
>Out[79]=    {2,4,6}
>Now try this:
>In[80]:=     {1,2,3}/.(m_List->(2*#& /@ m))
>Out[80]=    {1,2,3}
>Does anyone (say, at WRI for example) care to comment on
>Will Self

Tom Burton

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