Cheat sheet for common cats monad and fs2 operation shapes

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)`
attempt	`F[A]`	`F[Either[E, A]]`	Given `ApplicativeError[F, E]`
.	.	.	.
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.evalTap	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

Daenyth/MonadAndFs2Ops.md