September 2018
Intermediate to advanced
398 pages
9h 43m
English
Content preview from Scala Programming Projects
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Laws
In order to be Functor, the Functor instance's map function must satisfy two laws. In the rest of this chapter, we will define our laws in terms of an equality: left_expression == right_expression. These equalities must be true for any types and instances specified.
Given a type F that has a Functor[F] instance, for any type A and any instance fa: F[A], the following equalities must be satisfied:
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Identity preservation: fa.map(identity) == fa. The identity function always returns its argument. Mapping with this function should not change fa.
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Composition preservation: For any function f and g, fa.map(f).map(g) == fa.map(f andThen g). Mapping f and g successively is the same as mapping with a composition of these functions. This law ...
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