string—a string.
eof-error-p—a generalized boolean. The default is true.
eof-value—an object.
The default is nil.
start, end—bounding index designators of string.
The defaults for start and end are 0 and nil, respectively.
preserve-whitespace—a generalized boolean. The default is false.
object—an object (parsed by the Lisp reader) or the eof-value.
position—an integer greater than or equal to zero, and less than or equal to one more than the length of the string.
Parses the printed representation of an object
from the subsequence of string bounded by start and end,
as if read had been called on an input stream
containing those same characters.
If preserve-whitespace is true, the operation will preserve whitespace2
as read-preserving-whitespace would do.
If an object is successfully parsed, the primary value, object, is the object that was parsed. If eof-error-p is false and if the end of the substring is reached, eof-value is returned.
The secondary value, position, is the index of the first character in the bounded string that was not read. The position may depend upon the value of preserve-whitespace. If the entire string was read, the position returned is either the length of the string or one greater than the length of the string.
(read-from-string " 1 3 5" t nil :start 2) → 3, 5 (read-from-string "(a b c)") → (A B C), 7
If the end of the supplied substring occurs before an object can be read, an error is signaled if eof-error-p is true. An error is signaled if the end of the substring occurs in the middle of an incomplete object.
read, read-preserving-whitespace
The reason that position is allowed to be beyond the length of the string is to permit (but not require) the implementation to work by simulating the effect of a trailing delimiter at the end of the bounded string. When preserve-whitespace is true, the position might count the simulated delimiter.