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  ANSI Common Lisp   2 Syntax   2.3 Interpretation of Tokens   2.3.1 Numbers as Tokens Potential Numbers as Tokens

To allow implementors and future Common Lisp standards to extend the syntax of numbers, a syntax for potential numbers is defined that is more general than the syntax for numbers. A token is a potential number if it satisfies all of the following requirements:

  • 1. The token consists entirely of digits, signs, ratio markers, decimal points (.), extension characters (^ or _), and number markers. A number marker is a letter. Whether a letter may be treated as a number marker depends on context, but no letter that is adjacent to another letter may ever be treated as a number marker. Exponent markers are number markers.

  • 2. The token contains at least one digit. Letters may be considered to be digits, depending on the current input base, but only in tokens containing no decimal points.

  • 3. The token begins with a digit, sign, decimal point, or extension character, but not a package marker. The syntax involving a leading package marker followed by a potential number is not well-defined. The consequences of the use of notation such as :1, :1/2, and :2^3 in a position where an expression appropriate for read is expected are unspecified.

  • 4. The token does not end with a sign.

If a potential number has number syntax, a number of the appropriate type is constructed and returned, if the number is representable in an implementation. A number will not be representable in an implementation if it is outside the boundaries set by the implementation-dependent constants for numbers. For example, specifying too large or too small an exponent for a float may make the number impossible to represent in the implementation. A ratio with denominator zero (such as -35/000) is not represented in any implementation. When a token with the syntax of a number cannot be converted to an internal number, an error of type reader-error is signaled. An error must not be signaled for specifying too many significant digits for a float; a truncated or rounded value should be produced.

If there is an ambiguity as to whether a letter should be treated as a digit or as a number marker, the letter is treated as a digit.  Escape Characters and Potential Numbers  Examples of Potential Numbers

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