TIP #232 Version 1.1: Creating New Math Functions for the 'expr' Command

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes to expose the Tcl_CreateMathFunc() function to the script level, so that programmers can define new math functions using Tcl procedures.

Rationale

It has long been possible to define new (mathematical) functions for the expr command via the C API - the function Tcl_CreateMathFunc() is meant for this. However, it is not possible to define new math functions at the script level, so that you could use a Tcl procedure instead of a C function.

There exist one or two extensions that allow this, most notably "funcproc" by D. Fellows - <http://wiki.tcl.tk/541>), but as the Tcl core does not allow it, it remains a somewhat hidden feature. The current TIP aims at making the functionality of such extensions standard by introducing a new command.

With "scripted" math functions you can simplify such complex and error-prone expressions as:

  set max [expr {$max>($a+1)? $max : ($a+1)}]

or

  set f [expr { $x*[J0 $x] + [J1 [expr {2.0*$x}] }]

to the more regular:

  set max [expr {max($max,$a+1)}]

and

  set f [expr { $x*J0($x) + J1(2.0*$x) }]

(assuming the additional functions max, J0 and J1 have been defined of course).

To quote Donal Fellows:

"If you are interested in going the other way - writing functions for expr in Tcl without the use of square brackets (useful for when you want to expose expressions to users without having to tell them to learn all about Tcl, no matter how much that would benefit them in the long run. This sort of situation also requires the use of safe interpreters) - then you will probably find http://www.man.ac.uk/~zzcgudf/tcl/#scripts/funcproc very interesting indeed."

Proposed New Command

The functionality hinted at above can be implemented by a single new command:

exprfunc funcname procname

where:

• funcname is the name of the function as it is known to the expr command

• procname is the Tcl procedure that implements the command

The implementation is to be such that:

• Use is made of the object-style interface to avoid unnecessary string conversions

• The result of the function is either well-defined or an error is raised if the procedure raises an error

There are a few restrictions to procedures used in this way, as the expr command does not deal well with functions that can have a variable number of arguments: they can not use the magical "args" argument or optional arguments.

Reference Implementation

A reference implementation does not yet exist, but Donal Fellows's funcproc code is a good starting point.

Copyright

This document is placed in the public domain.

This is not necessarily the current version of this TIP.