This variable is intended primarily to support the operation of the
read-time conditional notations #+ and #-. It is important for the
reader macros which implement these notations
to be able to skip over the printed representation of an
expression despite the possibility that the syntax of the skipped
expression may not be entirely valid for the current implementation,
since #+ and #- exist in order to allow the same program to be
shared among several Lisp implementations (including dialects other than Common Lisp)
despite small incompatibilities of syntax.
If it is false, the Lisp reader operates normally.
If the value of *read-suppress* is true, read, read-preserving-whitespace, read-delimited-list, and read-from-string all return a primary value of nil when they complete successfully; however, they continue to parse the representation of an object in the normal way, in order to skip over the object, and continue to indicate end of file in the normal way. Except as noted below, any standardized reader macro_2 that is defined to read_2 a following object or token will do so, but not signal an error if the object read is not of an appropriate type or syntax. The standard syntax and its associated reader macros will not construct any new objects (e.g., when reading the representation of a symbol, no symbol will be constructed or interned).
All extended tokens are completely uninterpreted. Errors such as those that might otherwise be signaled due to detection of invalid potential numbers, invalid patterns of package markers, and invalid uses of the dot character are suppressed.
Dispatching macro characters continue to parse an infix numerical argument, and invoke the dispatch function. The standardized sharpsign reader macros do not enforce any constraints on either the presence of or the value of the numerical argument.
The #= notation is totally ignored. It does not read
a following object. It produces no object,
but is treated as whitespace_2.
The ## notation always produces nil.
No matter what the value of *read-suppress*,
parentheses still continue to delimit and construct lists;
the #( notation continues to delimit vectors;
and comments, strings,
and the single-quote and backquote notations continue to be
interpreted properly. Such situations as
'), #<,
#), and #<Space> continue to signal errors.
(let ((*read-suppress* t))
(mapcar #'read-from-string
'("#(foo bar baz)" "#P(:type :lisp)" "#c1.2"
"#.(PRINT 'FOO)" "#3AHELLO" "#S(INTEGER)"
"#*ABC" "#\GARBAGE" "#RALPHA" "#3R444")))
⇒ (NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL)
Programmers and implementations that define additional macro characters are strongly encouraged to make them respect *read-suppress* just as standardized macro characters do. That is, when the value of *read-suppress* is true, they should ignore type errors when reading a following object and the functions that implement dispatching macro characters should tolerate nil as their infix parameter value even if a numeric value would ordinarily be required.