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MzScheme can read and print graphs, S-expressions with
shared structure (e.g., a cycle). Graphs are described by tagging the
shared structure once with #n= (using some decimal integer
n with no more than eight digits) and then referencing it later with
#n# (using the same number n). For example, the
following S-expression describes the infinite list of ones:
#0=(1 . #0#)
If this graph is entered into MzScheme's read-eval-print loop,
MzScheme's compiler will loop forever, trying to
compile an infinite expression. In contrast, the following expression
defines ones to the infinite list of ones, using quote to
hide the infinite list from the compiler:
(define ones (quote #0=(1 . #0#)))
A tagged structure can be referenced multiple
times. Here, v is defined to be a vector containing the same
cons cell in all three slots:
(define v #(#1=(cons 1 2) #1# #1#))
A tag #n= must appear to the left of all references
#n#, and all references must appear in the same top-level
S-expression as the tag. By default, MzScheme's printer will display a
value without showing the shared structure:
#((1 . 2) (1 . 2) (1 . 2))
Graph reading and printing are controlled with the read-accept-graph and print-graph Boolean parameters (see section 9.4.1.4). Graph reading is enabled by default, and graph printing is disabled by default. However, when the printer encounters an graph containing a cycle, graph printing is automatically enabled (temporarily). When graph reading is disabled and a graph is provided as input, the exn:read exception is raised.
If the n in a #n= form or a #n# form contains more than eight digits, the exn:read exception is raised. If a #n# form is not preceded by a #n= form using the same n, the exn:read exception is raised. If two #n= forms are in the same expression for the same n, the exn:read exception is raised.