TIP #85 Version 1.1: Custom Comparisons in Tcltest

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a simple mechanism to make the tcltest package an even more flexible package than it already is by allowing the programmer to define his or her own comparison procedures. Such procedures can deal with issues like allowing a (small) tolerance in floating-point results.

Rationale

The most recent version of the tcltest package supports the comparison of the actual result with the expected result by a number of methods: exact matching, glob-style matching and matching via a regular expression. The flexibility is indeed enhanced as it is now much easier to allow for small variations in string results. But it is nearly impossible to define an accurate test that checks if floating-point results are the "same" - exact matching will seldom suffice due to platform-specific round-off errors or differences in formatting a floating-point number (0.12 versus .12 for instance).

It is also impossible to compare results that are not easily expressed as strings, for instance an application that produces binary files that need to be compared or simply very long strings - these could easily be stored in an external file, but would be awkward in a file with a large number of such tests.

Proposal

The tcltest package defines a comparison procedure, stringCompare, which takes care of the three cases. This procedure can easily be enhanced by adding a fourth case: invoking a custom procedure. This procedure takes two arguments and returns 1 for a match and a 0 for failure, just like the other cases.

Such a procedure can be registered by a simple mechanism:

 ::tcltest::customMatchProc mymatchproc

and will be invoked when the match type is set to "custom", so one has to specify this per test case, just as in the current situation. The default method will still be "exact".

Two Examples

To show how this works, we include two simple examples:

• Testing a package for calculating mathematical functions like Bessel functions.

• Testing for negative results, as when providing an alternative, but incompatible implementation of a feature.

First, suppose you have defined a package for calculating the value of a general Bessel function, just the sort of function that returns floating-point numbers. Then the results may be imprecise due to rounding-off errors, different values of tcl_precision or, even more banally, differences in the formatting of floating-point numbers (0.12 versus .12 for instance). A comparison procedure like:

 proc matchFloat { actual expected } {
    set absmax [expr {abs($actual)>abs($expected)? \
       abs($actual) : abs($expected)}
    expr {abs($actual-$expected)<=1.0e-15*$absmax}
 }

would help to solve this issue.

The second example occurs for instance when testing alternative implementation: you want to check that the original standard feature is failing whereas the new but incompatible alternative gets it right. Then:

 proc matchNegative { actual expected } {
    expr {![string equal $actual $expected]}
 }

makes sure that a mismatch is treated as the expected outcome.

Alternatives

Of course, it is possible to achieve these effects within the current framework of tcltest, by putting these match procedures inside the body of the test case. There are at least two drawbacks to this approach:

• The result against which we want to match is hidden in the code

• If the test fails, the actual result is not printed (at least not by the tcltest framework).

Copyright

This document is placed in the public domain.

This is not necessarily the current version of this TIP.