Cheat sheet for common cats monad and fs2 operation shapes

MonadAndFs2Ops.md

Operation	Input	Result	Notes
map	F[A] , A => B	F[B]	Functor
apply	F[A] , F[A => B]	F[B]	Applicative
(fa, fb, ...).mapN	(F[A], F[B], ...) , (A, B, ...) => C	F[C]	Applicative
(fa, fb, ...).tupled	(F[A], F[B], ...)	F[(A, B, ...)]	Applicative
flatMap	F[A] , A => F[B]	F[B]	Monad
traverse	F[A] , A => G[B]	G[F[A]]	Traversable; fa.traverse(f) == fa.map(f).sequence; "foreach with effects"
sequence	F[G[A]]	G[F[A]]	Same as fga.traverse(identity)
.	.	.	.
Stream.emit	A	Stream[Pure, A]
Stream.emits	Iterable[A]	Stream[Pure, A]
stream.covary[F]	Stream[Pure, A]	Stream[F, A]	Often useful after emit or emits. ; F is anything with cats-effect Effect[F] available. Usually cats.effect.IO
Stream.eval	F[A]	Stream[F, A]	F: Effect
stream.evalMap	Stream[F, A] , A => F[B]	Stream[F, B]	Run an effect for each element in the stream, returning each result
stream.observe1	Stream[F, A], A => F[Unit]	Stream[F, A]	Run an effect for each element in the stream, ignoring the result
stream.compile.drain	Stream[F, A]	F[Unit]	Translate the stream description to a description of effects
stream.compile.toVector	Stream[F, A]	F[Vector[A]]	Translate the stream description to a description of effects returning a vector