ANSI Common Lisp 22 Printer 22.3 Formatted Output 22.3.3 FORMAT Floating-Point Printers
188.8.131.52 Tilde G: General Floating-Point
The next arg is printed as a float in either fixed-format or exponential notation as appropriate.
The full form is ~w,d,e,k,overflowchar,padchar,exponentcharG. The format in which to print arg depends on the magnitude (absolute value) of the arg. Let n be an integer such that 10n-1 <= |arg| < 10n. Let ee equal e+2, or 4 if e is omitted. Let ww equal w-ee, or nil if w is omitted. If d is omitted, first let q be the number of digits needed to print arg with no loss of information and without leading or trailing zeros; then let d equal (max q (min n 7)). Let dd equal d-n.
If 0 <= dd <= d, then arg is printed as if by the format directives
Note that the scale factor k is not passed to the ~F directive. For all other values of dd, arg is printed as if by the format directive
In either case, an @ modifier is supplied to the ~F or ~E directive if and only if one was supplied to the ~G directive.