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Last active June 25, 2024 13:05
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Scala (cats) map/traverse parallels
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1-MapTraverse.md

Parallels between map and similar functions

map          :: F[A] => (A =>     B)   => F[B]
flatMap      :: F[A] => (A =>   F[B])  => F[B]
traverse     :: G[A] => (A =>   F[B])  => F[G[B]]
flatTraverse :: G[A] => (A => F[G[B]]) => F[G[B]]
traverse_    :: F[A] => (A =>   F[B])  => F[Unit]
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2-TraverseSequence.md

traverse vs sequence

Both let you "flip" nested effects inside out

traverse  :: G[A] => (A => F[B]) => F[G[B]]
sequence  :: G[F[A]]             => F[G[A]]

You can also run just the effects and ignore the value

traverse_ :: F[A] => (A => F[B]) => F[Unit]
sequence_ :: G[F[A]]             => F[Unit]

Both functions can be defined in terms of each other

fa.traverse(f) == fa.map(f).sequence
fa.sequence    == fa.traverse(identity)
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