2.2 Reader Algorithm

This section describes the algorithm used by the Lisp reader to parse objects from an input character stream, including how the Lisp reader processes macro characters.

When dealing with tokens, the reader's basic function is to distinguish representations of symbols from those of numbers. When a token is accumulated, it is assumed to represent a number if it satisfies the syntax for numbers listed in Figure 2.9. If it does not represent a number, it is then assumed to be a potential number if it satisfies the rules governing the syntax for a potential number. If a valid token is neither a representation of a number nor a potential number, it represents a symbol.

The algorithm performed by the Lisp reader is as follows:

1. If at end of file, end-of-file processing is performed as specified in read. Otherwise, one character, x, is read from the input stream, and dispatched according to the syntax type of x to one of steps 2 to 7.
2. If x is an invalid character, an error of type reader-error is signaled.
3. If x is a whitespace₂ character, then it is discarded and step 1 is re-entered.
4. If x is a terminating or non-terminating macro character then its associated reader macro function is called with two arguments, the input stream and x.

   The reader macro function may read characters from the input stream; if it does, it will see those characters following the macro character. The Lisp reader may be invoked recursively from the reader macro function.

   The reader macro function must not have any side effects other than on the input stream; because of backtracking and restarting of the read operation, front ends to the Lisp reader (e.g., “editors” and “rubout handlers”) may cause the reader macro function to be called repeatedly during the reading of a single expression in which x only appears once.

   The reader macro function may return zero values or one value. If one value is returned, then that value is returned as the result of the read operation; the algorithm is done. If zero values are returned, then step 1 is re-entered.

5. If x is a single escape character then the next character, y, is read, or an error of type end-of-file is signaled if at the end of file. y is treated as if it is a constituent whose only constituent trait is alphabetic₂. y is used to begin a token, and step 8 is entered.
6. If x is a multiple escape character then a token (initially containing no characters) is begun and step 9 is entered.
7. If x is a constituent character, then it begins a token. After the token is read in, it will be interpreted either as a Lisp object or as being of invalid syntax. If the token represents an object, that object is returned as the result of the read operation. If the token is of invalid syntax, an error is signaled. If x is a character with case, it might be replaced with the corresponding character of the opposite case, depending on the readtable case of the current readtable, as outlined in Section 23.1.2 (Effect of Readtable Case on the Lisp Reader). X is used to begin a token, and step 8 is entered.
8. At this point a token is being accumulated, and an even number of multiple escape characters have been encountered. If at end of file, step 10 is entered. Otherwise, a character, y, is read, and one of the following actions is performed according to its syntax type:
   • If y is a constituent or non-terminating macro character:
     • If y is a character with case, it might be replaced with the corresponding character of the opposite case, depending on the readtable case of the current readtable, as outlined in Section 23.1.2 (Effect of Readtable Case on the Lisp Reader).
     • Y is appended to the token being built.
     • Step 8 is repeated.
   • If y is a single escape character, then the next character, z, is read, or an error of type end-of-file is signaled if at end of file. Z is treated as if it is a constituent whose only constituent trait is alphabetic₂. Z is appended to the token being built, and step 8 is repeated.
   • If y is a multiple escape character, then step 9 is entered.
   • If y is an invalid character, an error of type reader-error is signaled.
   • If y is a terminating macro character, then it terminates the token. First the character y is unread (see unread-char), and then step 10 is entered.
   • If y is a whitespace₂ character, then it terminates the token. First the character y is unread if appropriate (see read-preserving-whitespace), and then step 10 is entered.
9. At this point a token is being accumulated, and an odd number of multiple escape characters have been encountered. If at end of file, an error of type end-of-file is signaled. Otherwise, a character, y, is read, and one of the following actions is performed according to its syntax type:
   • If y is a constituent, macro, or whitespace₂ character, y is treated as a constituent whose only constituent trait is alphabetic₂. Y is appended to the token being built, and step 9 is repeated.
   • If y is a single escape character, then the next character, z, is read, or an error of type end-of-file is signaled if at end of file. Z is treated as a constituent whose only constituent trait is alphabetic₂. Z is appended to the token being built, and step 9 is repeated.
   • If y is a multiple escape character, then step 8 is entered.
   • If y is an invalid character, an error of type reader-error is signaled.
10. An entire token has been accumulated. The object represented by the token is returned as the result of the read operation, or an error of type reader-error is signaled if the token is not of valid syntax.