Why it matters

Cats Effect underlies major FP libraries. Understanding shapes app design.

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The architecture

IO[A]: effect producing A.

Resource[F, A]: acquire+release safely.

Concurrency: Fiber (like green thread), Ref (concurrent state).

Cats Effect building blocksIOeffectResourcesafe acquire+releaseFiber + Refconcurrency primitivesComposes with Cats type classes for full functional Scala experience
Cats Effect primitives.
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How it works end to end

Fibers: lightweight concurrent computations. Compose via join, race.

Ref: thread-safe mutable reference.

Deferred: one-time signal.

Ecosystem: http4s (HTTP), fs2 (streaming), doobie (DB), skunk (Postgres).