*read-default-float-format*

∗read-default-float-format∗ Variable

Value Type:

one of the atomic type specifiers short-float, single-float, double-float, or long-float, or else some other type specifier defined by the implementation to be acceptable.

Initial Value:

The symbol single-float.

Description:

Controls the floating-point format that is to be used when reading a floating-point number that has no exponent marker or that has e or E for an exponent marker . Other exponent markers explicitly prescribe the floating-point format to be used.

The printer uses *read-default-float-format* to guide the choice of exponent markers when printing floating-point numbers.

Examples:

(let ((\*read-default-float-format\* ’double-float)) 
  (read-from-string "(1.0 1.0e0 1.0s0 1.0f0 1.0d0 1.0L0)")) 
→ (1.0 1.0 1.0 1.0 1.0 1.0) ;Implementation has float format F. 
→ (1.0 1.0 1.0s0 1.0 1.0 1.0) ;Implementation has float formats S and F. → (1.0d0 1.0d0 1.0 1.0 1.0d0 1.0d0) ;Implementation has float formats F and D. → (1.0d0 1.0d0 1.0s0 1.0 1.0d0 1.0d0) ;Implementation has float formats S, F, D. → (1.0d0 1.0d0 1.0 1.0 1.0d0 1.0L0) ;Implementation has float formats F, D, L. → (1.0d0 1.0d0 1.0s0 1.0 1.0d0 1.0L0) ;Implementation has formats S, F, D, L. 
*∗***read-eval***∗ Variable* 

Value Type:

a generalized boolean.

Initial Value:

true.

Description:

If it is true, the #. reader macro has its normal effect. Otherwise, that reader macro signals an error of type reader-error.

See Also:

*print-readably*

Notes:

If *read-eval* is false and *print-readably* is true, any method for print-object that would output a reference to the #. reader macro either outputs something different or signals an error of type print-not-readable.

Expanded Reference: *read-default-float-format*

Default behavior

*read-default-float-format* controls the type of float produced when reading a floating-point number that has no exponent marker or uses e/E. Its initial value is single-float.

*read-default-float-format*
=> SINGLE-FLOAT

(type-of (read-from-string "1.0"))
=> SINGLE-FLOAT

Changing the default float format

By binding *read-default-float-format* to double-float, numbers without an explicit exponent marker are read as double floats.

(let ((*read-default-float-format* 'double-float))
  (type-of (read-from-string "1.0")))
=> DOUBLE-FLOAT

(let ((*read-default-float-format* 'double-float))
  (type-of (read-from-string "3.14e0")))
=> DOUBLE-FLOAT

Explicit exponent markers override the default

The exponent markers s, f, d, and l always specify their corresponding float types regardless of *read-default-float-format*.

(let ((*read-default-float-format* 'double-float))
  (list (type-of (read-from-string "1.0"))     ; uses default
        (type-of (read-from-string "1.0s0"))   ; short-float
        (type-of (read-from-string "1.0f0"))   ; single-float
        (type-of (read-from-string "1.0d0"))   ; double-float
        (type-of (read-from-string "1.0l0")))) ; long-float
=> (DOUBLE-FLOAT SHORT-FLOAT SINGLE-FLOAT DOUBLE-FLOAT LONG-FLOAT)
;; Note: some implementations merge certain float types

Effect on the printer

*read-default-float-format* also influences the printer: when printing a float whose type matches the default format, the printer may omit the exponent marker or use E.

(let ((*read-default-float-format* 'single-float))
  (format nil "~S ~S" 1.0 1.0d0))
=> "1.0 1.0d0"

(let ((*read-default-float-format* 'double-float))
  (format nil "~S ~S" 1.0 1.0d0))
=> "1.0f0 1.0"
;; The double-float is now the default so it drops the marker;
;; the single-float must use f0 to distinguish itself