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Document Lisp floats a bit better

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* doc/lispref/numbers.texi (Float Basics):
* doc/misc/cl.texi (Implementation Parameters):
* lisp/emacs-lisp/cl-lib.el (cl-most-positive-float)
(cl-least-positive-float)
(cl-least-positive-normalized-float, cl-float-epsilon)
(cl-float-negative-epsilon):
Document IEEE floating point better.  Don’t suggest that Emacs
might use some floating-point format other than IEEE format, as
Emacs currently assumes IEEE in several places and there seems
little point in removing those assumptions.
This commit is contained in:
Paul Eggert 2019-11-10 15:01:06 -08:00
parent 6ad5eb9794
commit b6942c0c37
3 changed files with 37 additions and 41 deletions
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16
lisp/emacs-lisp/cl-lib.el
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@ -299,7 +299,7 @@ If true return the decimal value of digit CHAR in RADIX."
(defconst cl-most-positive-float nil
"The largest value that a Lisp float can hold.
If your system supports infinities, this is the largest finite value.
For IEEE machines, this is approximately 1.79e+308.
For Emacs, this equals 1.7976931348623157e+308.
Call `cl-float-limits' to set this.")
(defconst cl-most-negative-float nil
@ -309,8 +309,8 @@ Call `cl-float-limits' to set this.")
(defconst cl-least-positive-float nil
"The smallest value greater than zero that a Lisp float can hold.
For IEEE machines, it is about 4.94e-324 if denormals are supported,
or 2.22e-308 if they are not.
For Emacs, this equals 5e-324 if subnormal numbers are supported,
`cl-least-positive-normalized-float' if they are not.
Call `cl-float-limits' to set this.")
(defconst cl-least-negative-float nil
@ -320,10 +320,8 @@ Call `cl-float-limits' to set this.")
(defconst cl-least-positive-normalized-float nil
"The smallest normalized Lisp float greater than zero.
This is the smallest value for which IEEE denormalization does not lose
precision. For IEEE machines, this value is about 2.22e-308.
For machines that do not support the concept of denormalization
and gradual underflow, this constant equals `cl-least-positive-float'.
This is the smallest value that has full precision.
For Emacs, this equals 2.2250738585072014e-308.
Call `cl-float-limits' to set this.")
(defconst cl-least-negative-normalized-float nil
@ -334,12 +332,12 @@ Call `cl-float-limits' to set this.")
(defconst cl-float-epsilon nil
"The smallest positive float that adds to 1.0 to give a distinct value.
Adding a number less than this to 1.0 returns 1.0 due to roundoff.
For IEEE machines, epsilon is about 2.22e-16.
For Emacs, this equals 2.220446049250313e-16.
Call `cl-float-limits' to set this.")
(defconst cl-float-negative-epsilon nil
"The smallest positive float that subtracts from 1.0 to give a distinct value.
For IEEE machines, it is about 1.11e-16.
For Emacs, this equals 1.1102230246251565e-16.
Call `cl-float-limits' to set this.")