Author: Kevin B. Kenny <firstname.lastname@example.org> State: Draft Type: Project Vote: Pending Created: 18 October 2018 Post-History: Tcl-Version: 8.7 Keywords: Tcl, floating point, NaN, not a number Tcl-Branch: tip-520
This TIP proposes that the special floating point constants
) be accepted in any context where
other floating point constants may appear. This change has the effect
of changing Tcl from 'signalling NaN' semantics (where floating-point
exceptions are errors) to 'quiet NaN' semantics (where floating-point
exceptions return the constant,
NaN, and operations where one of the
NaN propagate the
NaN to their results).
Since Tcl 8.5, [TIP 132] Tcl has had the capability to
recognize the special IEEE floating point constants,
(representing an infinity or an overflow) and
NaN (representing a
value that is 'Not a Number', ordinarily the result of some sort of
floating point exception). Infinities are now accepted on an equal
footing with other floating point constants; they can be passed
anywhere that such a constant can appear, and they follow IEEE 754
rules for arithmetic.
NaN is different. If a floating-point expression
NaN, or if
Tcl_GetDoubleFromObj receives such a
constant, the system treats the condition as an error, '
argument not in valid range'.
This behaviour was specified explicitly in paragraph 5 of TIP 132:
The input and output conversions shall allow for the IEEE special values
NaN(and for denormalized numbers). The [expr] command shall be changed to allow +Inf and -Inf as the result of an expression;
NaNshall still cause an error.
-Infas it does any ordinary floating point number, and return an error for
This sort of handling was adopted because at the time,
NaN was a
relatively new thing, and programmers were suspicious of it. Many
believed that if operations like 0./0. or asin(2.0) were allowed
NaN rather than throwing an immediate error, debugging
would be greatly complicated, to very little benefit. (The personal
recollection of the author of this TIP was that even infinities were
tough to sell, for much the same reason.)
NaN values have been much more broadly accepted. Most
C compilers have quiet-
NaN behaviour by default; a floating-point
NaN rather than raising
SIGFPE. The world
seems to have accepted William Kahan's remarks:
My thesis is that exceptions are not errors unless they are handled badly. Exceptions are opportunities for extra computation.... An exception is an event for which any policy you choose in advance will subsequently be found disadvantageous for somebody who will then, for good reason, take exception to the policy.
This change in sentiment was underlined for the author of this TIP
by the fact that in no fewer than three separate conversations at the
2018 Tcl/Tk Conference, he was approached with queries about
handling. An increasing number of users are dealing with external systems
NaN to indicate a missing value in a data set, and
NaN to be passed in and out gracefully.
Tcl's expression evaluator, and the functions in
::tcl::mathop, shall be changed to allow
NaN values to
be returned from floating-point expressions. (This change has already
been available in the source code for a long time, and can be enabled
-DACCEPT_NAN to the compilation options.)
The math functions
min shall be defined so that if any
argument to the function is
NaN, the output shall be
Functions requiring integers, functions coercing 'double' values to
entier), and the
function shall continue to throw errors when presented with
(there seems to be nothing sensible to do with these cases).
The C interface
Tcl_GetDoubleFromObj shall be modified to accept
NaN's without thowing errors.
The implementations of the
lsort -real and
lsearch -real commands
shall be modified to continue to throw errors if a
encountered in either the list or the search pattern. Since
unordered with respect to any floating point constant, there is no
correct behaviour for these commands when asked to sort a list
Most of the implementation has been in the code base for along time,
albeit hidden, under the
-DACCEPT_NAN compilation option.
A few changes (updates to the test suite to reflect the new default;
Tcl_GetDoubleFromObj; changes to
still need to be made; the link to the implementation branch will
be added to this TIP.
The change to
Tcl_GetDoubleFromObj, alone among the changes outlined
in this proposal, may need to be delayed until Tcl 9. It introduces a
subtle incompatibility in C code in that quiet
NaN values can be
returned to a caller that does not expect them. In particular, the
author has not yet investigated whether Tk methods such as
yscroll will survive such exceptional
arguments without throwing
SIGFPE. These are invalid arguments -
the author is willing to accept almost any behaviour that does not
render the UI unusable or abort the process, but we need to verify
.my.widget yview NaN does not invoke nasal daemons.