Applicative

(Note - don’t use this, use cats)

Simple implementation of an Applicative type-class. There are many choices for the canonical second operation (join, sequence, joinWith, ap), all equivalent. For a Functor modeling concurrent computations with failure, like Future, combining results with join can save a lot of time over combining with flatMap.

If your Error type is a singleton, given two operations, if the second fails before the first completes, one can fail the entire computation right then. With flatMap, one would have to wait for the first operation to complete before failing it. If your Error type isn’t a singleton, if you want your ap / zip to agree with flatMap, you have to wait on the first operation to fail, even if the second fails, so that you can surface the error from the first. If the first fails, then you can bail out of the 2nd one early, but not vice versa.

Laws Applicative instances must follow:

map(apply(x))(f) == apply(f(x))
join(apply(x), apply(y)) == apply((x, y))

(sequence and joinWith specialize join - they should behave appropriately.)

Documentation Help

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• Applicative.scala
• ApplicativeProperties.scala