Lehman's Laws of Software Evolution - Context

Context

Lehman and Belady classified programs into three types:

  • S-type programs are those that can be specified formally.
  • P-type programs cannot be specified. Instead, an iterative process is used to find a working solution.
  • E-type programs are embedded in the real world and become part of it, thereby changing it. This leads to a feedback system where the program and its environment evolve in concert.

The laws of software evolution were originally based on observations regarding the evolution of IBM's OS/360 and OS/370. The laws were not presented as laws of nature, but rather as general observations that are expected to hold for all E-type systems, regardless of specific programming or management practices.

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