The signatures of pure functions in FP applications are crucial because they reveal exactly what the functions do without needing to inspect the source code.
Unlike OOP methods that can have side effects, pure functions have no side effects, making their signatures much more meaningful and trustworthy.
In OOP, a method's pure or impure nature is often opaque; for instance, what appears to be a simple setter can perform multiple unexpected tasks.
Pure functions in FP have a clear relationship between inputs and outputs, as all values are immutable, ensuring predictability and reliability.
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