Anonymous recursion: Difference between revisions
Added Quackery.
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(A A N)))
</lang>
=={{header|Quackery}}==
Quackery can do anonymous recursion via the word <code>recurse</code>.
<pre>[ dup 0 < iff
$ "negative argument passed to fibonacci"
fail
[ dup 2 < if done
dup 1 - recurse
swap 2 - recurse + ] ] is fibonacci ( n --> n )</pre>
<code>recurse</code> causes the current nest (i.e. the on that starts <code>[ dup</code> and ends <code>+ ]</code> to be evaluated recursively. This means that if, for example, the first recursive call was inside one or more nested nests, it would not work as desired. So the first instance of <code>recurse</code> in:
<pre>( *** faulty code *** )
[ dup 0 < iff
$ "negative argument passed to fibonacci"
fail
[ dup 2 < if done
[ [ [ [ [ [
[ dup 1 - recurse ]
] ] ] ] ] ]
swap 2 - recurse + ] ] is fibonacci ( n --> n )</pre>
would cause the nest <code>[ dup 1 - recurse ]</code>to be evaluated, and the program would go into a tailspin, recursively evaluating that nest until the return stack overflowed.
This limitation can be overcome with the understanding that recursion can be factored out into two ideas, i.e. self=reference and evaluation. The self-reference word in Quackery is <code>this</code>, which puts a pointer to the current nest on the data stack (usually just called "the stack") and the evaluation word is <code>do</code>, which takes a pointer from the stack and evaluates it. So <code>this do</code> is equivalent to <code>recurse</code>.
The final example fixes the previous example by putting <code>this</code> at the correct level of nesting and evaluating it within the nested nests with <code>do</code>.
<pre>[ dup 0 < iff
$ "negative argument passed to fibonacci"
fail
[ dup 2 < if done
this
[ [ [ [ [ [
[ dip [ dup 1 - ] do ]
] ] ] ] ] ]
swap 2 - recurse + ] ] is fibonacci ( n --> n )
</pre>
Quackery also provides named recursion with <code>forward</code> and <code>resolves</code>, so this is usually not ''necessary'' but can be useful in unusual circumstances.
=={{header|R}}==
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