Re: Recursive anonymous functions?
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- Subject: Re: Recursive anonymous functions?
- From: Gabor Grothendieck <ggrothendieck@...>
- Date: 2004年7月22日 17:49:05 +0000 (UTC)
Philip Bock <phil <at> flamewars.org> writes:
> Is there any way for an anonymous function to call itself?
It seems your problem can be answered without addressing the
literal question you asked but just for the sake of
completing this, we could use fixed point combinator ideas
from computer science to create anonymous recursive
functions.
Try this:
1. define this utility function:
Y = function(f) return f(f) end
2. define an anonymous function which is a function with
argument f, say, that returns a function that is the same as
the factorial function except that f(f) is written in place
of the recursive function call.
3. pass the anonymous function in #2 to Y. Y takes #2
as an input and outputs the desired recursive factorial
function.
For example,
Y = function(f) return f(f) end
print(Y(function(f)
return function(n)
if (n == 0) then
return 1
else
return n * f(f)(n-1)
end
end
end)(3))
will print 3 factorial which is 6.
For more on this, check out
http://okmij.org/ftp/Computation/fixed-point-combinators.html
and google for fixed point combinator for even more.
I know of one language that has a built in Recall function that
will recursively call the function its called in to make it
easier to write anonymous recusrive functions. I don't think Lua
has such a facility but if it does someone might be able to point it
out.