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XLR: Extensible Language and Runtime

The art of turning ideas into code

Semantic Noise

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Semantic noise is a difference the semantics being used in the code and the concept it represents in the problem space. In other words, it is an indication that the code doesn't mean what a naive programmer might expect. It is an important metric for Concept Programming. Since this is a form of noise, it is desirable to reduce it, but it is in general impossible to completely eliminate it. And also remember that noise is somewhat subjective, so what is noise to one person may be music to another.

Here are a few examples of common forms of semantic noise:

  • Numbers in computers are limited compared to their mathematical counterparts. The range of integers is not infinite, and real numbers don't have infinite precision, to cite two simple examples. This creates many classical forms of semantic noise. For instance, using 16-bit signed computations, (32767+3)/2 is generally not 16385. The result depends on the language. In C, it is a negative value (-16383). In Ada, a NUMERIC_ERROR or RANGE_ERROR exception will be raised depending on the implementation. In that case, Ada makes the situation where noise exists even noisier, to make sure it is noticed.

    Similar issues exist for floating point numbers, where the value 1+x-1 is often 0 rather than x for very small values of x (because 1+x has been rounded to available precision). As an amusing aside, in C, the types representing real numbers are not called real but float and double. Both names indicate an implementation detail (floating-point and double precision)... but not the same one!

  • C uses the << operator to indicate left bit shifts, so in C out<<3 shifts the integral value out left by 3 bit positions (multiplying it by 8). C++ introduces the possibility to overload operators, and the C++ standard library uses the << operator with a totally different meaning, to output something to a stream. So now out<<3 may mean writing 3 into a file or on the console. There is a difference between the meaning a naive C programmer expects and the actual meaning. This is only a relatively minor problem with this notation, because it affects only C programmers and the compiler will detect misuses. Two more serious forms of semantics noise with C++ I/Os are discussed here (inability to be made thread-safe, and incorrect model of what a text line is).

See the concept programming analysis of existing languages for examples combining syntactic noise and semantic noise.

Semantic noise complicates maintenance of programs, and makes the learning curve steeper. It can cause bugs, because programmers tend to use their knowledge of the problem space as a model for the structure of the code, and semantic noise introduces a difference between the two structures. Concept casts are a frequent source of semantic noise.

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Copyright 2008 Christophe de Dinechin (Blog)
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