ANSI Common Lisp 2 Syntax 2.3 Interpretation of Tokens 2.3.2 Constructing Numbers from Tokens
188.8.131.52 Syntax of a FloatFloats can be written in either decimal fraction or computerized scientific notation: an optional sign, then a non-empty sequence of digits with an embedded decimal point, then an optional decimal exponent specification. If there is no exponent specifier, then the decimal point is required, and there must be digits after it. The exponent specifier consists of an exponent marker, an optional sign, and a non-empty sequence of digits. If no exponent specifier is present, or if the exponent marker e (or E) is used, then the format specified by *read-default-float-format* is used. See Figure 2.3.1 Numbers as Tokens.
An implementation may provide one or more kinds of float that collectively make up the type float. The letters s, f, d, and l (or their respective uppercase equivalents) explicitly specify the use of the types short-float, single-float, double-float, and long-float, respectively.
The internal format used for an external representation depends only on the exponent marker, and not on the number of decimal digits in the external representation.
The next figure contains examples of notations for floats: