Re: printing recursive objects?

Paul Haahr <haahr@mv.us.adobe.com> writes:


> For example, given the scheme expression
>
> (let ((x (cons 'a nil))) (set-cdr! x x) x)
>
> how do I print some scheme expression that, when interpreted,
> creates the same structure, rather than just a lot of ``a''s?


Chez Scheme has a parameter called print-graph. When enabled, write prints
objects in a notation which may be read in later. (Of course, this
requires the cooperation of read, as well.)


The notation is that the first occurence of a duplicated object is written
as `#n=object', and subsequent occurences of the object are written as
`#n#'. Your example would be printed as `#0=(a . #0#)'.


Of course a simple table lookup will suffice to detect duplicated objects.
I don't, unfortunately, have any references, or knowledge of more
complicated algorithms, if such exist. (The description in the Chez System
Manual is no more detailed than that the above.)
--

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