Writing a HoldAll function that evaluates its arguments
- To: mathgroup at smc.vnet.net
- Subject: [mg86198] Writing a HoldAll function that evaluates its arguments
- From: Szabolcs Horvát <szhorvat at gmail.com>
- Date: Thu, 6 Mar 2008 02:54:56 -0500 (EST)
- Organization: University of Bergen
What is the simplest and most elegant way of implementing a function
that has HoldAll, but it still evaluates each of its arguments exactly
once (and does not go into infinite evaluation)?
I can come up with some ways (explicitly checking whether evaluation
changes the arguments), but neither of them are particularly elegant.
The function should evaluate like this:
Attributes[f] === {HoldAll}
f[] --> f[]
f[3] --> f[3]
f[1+2] --> f[3]
f[1,2] --> f[1,2]
f[0+1,1+1] --> f[1,2]
etc.
Let me explain my motivation for asking this seemingly absurd question
with an example:
One built-in function that has this behaviour is And[]. This special
behaviour was needed to make And[] a "short-circuiting" function: it
evaluates the arguments one by one, and it returns False *immediately*
when one argument evaluates to False, without evaluating any further
arguments.
Szabolcs