abstraction

All categories are an imperfect depiction of reality. The truth always lies in the concrete.

Create abstractions from the concrete, to the degree they are useful for the task at hand.

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Abstract vs general

	Specific	General
Abstract	interface Stack<T> { push; pop } — hides the implementation of a stack only.	interface Monoid<T> { empty; combine } — a very high-level pattern many types fit (strings, numbers, sets…).
Concrete	class IntArrayStack — fixed array stack for int.	function sort<T>(xs: T[], cmp) — works for any T, but it’s concretely “arrays + a particular algorithm.”