TIP #174 Version 1.1: Math Operators as Commands

This is not necessarily the current version of this TIP.

Abstract

This TIP describes a proposal for math operators in Tcl as seperate commands, acting much like the equivalent in the Lisp language. This would make simple usage of mathematics much clearer and could possibly facilitate a clean path for future [expr] extendibility.

Rationale

While the [expr] command works fairly well for longer mathematical expressions, it is extremely tedious for the most common uses, such as handling indices. Take the following example:

 set newList [lrange $list [expr {$idx - 5}] [expr {$idx + 5}]]

Many find this particular aspect of Tcl unappealing. It gets increasingly difficult to read as more and more simple mathematical expressions build up.

In addition all functions related to mathematics are contained within the [expr] command with its own separate syntax and thus we effectively have two different kinds funtions with no obvious relation to one another: Tcl commands and mathematical functions. This has been shown to be quite confusing to a new user and decreases the elegance of Tcl as a whole.

Proposed Change

1. I propose a group of Tcl commands which would handle mathematical operations without the need to use [expr]. Each command would take a varying amount of arguments and would work such that the operator is applied to the combination of the first and second argument. The result of this combination is then used with the operator for the third argument etc. If only one argument is given, it is returned as is. A sample implementation of the + command in Tcl follows:

    proc + {r args} {
        foreach operand $args {
            set r [expr {$r + $operand}]
        }
        return $r
    }
   

2. The exceptions to rule number 1 are the operators ! (logical NOT), ~ (bitwise NOT) and x?y:z (if-then-else). The first two only accept one argument while if-then-else accepts three.

3. All operator commands will be kept in the tcl::math::ops namespace, from which they would most commonly be imported to the global namespace. Alternatively the math::ops namespace could be used.

4. Each [expr] mathematical function will be mirrored into a Tcl command of the same name with the same results as its [expr] counterpart.

5. Each mathematical command will be kept in the tcl::math namespace. Alternatively the math namespace could be used.

6. As a result of 4 and 5, current [expr] functions will be marked in the documentation as deprecated and that the Tcl commands should be used instead, with a warning in the documentation that they may be removed at some future time. They will, however, be kept in the language for as long as deemed necessary for purposes of backwards compatibility. [expr] functions offer no benefit over their command counterparts and only result in extra complexity and redundancy, which is something Tcl has done well to avoid.

7. The other aspects of the [expr] command will not be affected.

As an example use, let us change the lrange line from above:

 set newList [lrange $list [- $idx 5] [+ $idx 5]]

This is clearly shorter and much easier on the eyes. There is no need to consider the effects of bracing expressions.

It is not the purpose of this TIP to discuss the necessity of providing an interface for dynamic [expr] expansion, but with the above interface we could possibly build a mechanism for doing this, by adding new commands to the appropriate namespace. This would offer an alternative to TIP #133. Problems of precedence etc. that may result in such a facility are beyond the scope of this TIP.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.