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MathGroup Archive 2005

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  • To: mathgroup at smc.vnet.net
  • Subject: [mg53238] Thread
  • From: Carlo Teubner <"see signature."@example.com>
  • Date: Sun, 2 Jan 2005 04:12:47 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

I'm trying to write a variant of Thread which takes into account 
options. That is, I want

  ThreadOptions[ f[{1,2}, Opt1 -> {a,b}, Opt2 -> {x,y}] ]

to give

  {f[1, Opt1 -> a, Opt2 -> x], f[2, Opt1 -> b, Opt2 -> y]}.

A first approximation I've come up with is

  ThreadOptions[f_[args___, opts___?OptionQ]] :=
    Thread[f[args, Inner[Rule, Sequence@@Thread[{opts}, Rule], List]]]

Now I get

  {f[1, {Opt1 -> a, Opt2 -> x}], f[2, {Opt1 -> b, Opt2 -> y}]}

which is close. I need to get rid of the {}. The following works:

  ThreadOptions[f_[args___, opts___?OptionQ]] :=
    Thread[f[args, Inner[Rule, Sequence@@Thread[{opts}, Rule], seq]]] /.
     seq -> Sequence

However, there is a problem: now f gets evaluated before replacing the 
seq with Sequence objects. To get around this, I tried using Unevaluated:

  ThreadOptions[f_[args___, opts___?OptionQ]] :=
    Unevaluated[Thread[f[args, Inner[Rule, Sequence@@Thread[{opts},
    Rule], seq]]] /. seq -> Sequence

But it doesn't work; I get:

  Thread::tdlen : Objects of unequal length in f[{1, 2}, {Opt1 -> a,
  Opt2 -> x, Opt1 -> b, Opt2 -> y}] cannot be combined.

  f[{1, 2}, {Opt1 -> a, Opt2 -> x, Opt1 -> b, Opt2 -> y}]

I assume what's happened is that ReplaceAll has been applied to the 
Unevaluated expression, which has not been evaluated.

Is there any way at all of Thread'ing a function, but then not 
evaluating it? Are there other ways of creating a ThreadOptions function?

Thanks for any help.

Carlo


-- 
C   Teubner   t      offline   de
  dot        at  minus       dot
   ...oops: "on" not "off" :)


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