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KISS, DRY, YAGNI and the Principle of Least Surprise

SOLID and GRASP tell you how to assign responsibilities. KISS, DRY, and YAGNI tell you how much to build and how complex to make it. These three heuristics operate at a higher level than patterns — they are meta-principles that constrain the design process itself. The Principle of Least Surprise (also called Least Astonishment) ties them together: a codebase should behave in the way a competent developer expects, without hidden complexity, unnecessary duplication, or speculative features. Together, these four principles define the complexity budget of a healthy design.

ℹ️The goal of all four principles is the same: reduce the cognitive load required to understand, modify, and maintain a codebase. Complexity is not inherently bad — necessary complexity is the cost of solving hard problems. Accidental complexity — complexity introduced by poor design choices — is pure waste.
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