Introduction 6
1.2 Pure Functional Programming
Functional Programming is the act of writing programs with pure functions. Pure functions have
three properties:
• Total: return a value for every possible input
• Deterministic: return the same value for the same input
• Inculpable: no (direct) interaction with the world or program state.
Together, these properties give us an unprecedented ability to reason about our code. For example,
input validation is easier to isolate with totality, caching is possible when functions are deterministic,
and interacting with the world is easier to control, and test, when functions are inculpable.
The kinds of things that break these properties are side effects: directly accessing or changing
mutable state (e.g. maintaining a var in a class or using a legacy API that is impure), communicating
with external resources (e.g. files or network lookup), or throwing and catching exceptions.
We write pure functions by avoiding exceptions, and interacting with the world only through a safe
F[_] execution context.
In the previous section, we abstracted over execution and defined echo[Id] and echo[Future].
We might reasonably expect that calling any echo will not perform any side effects, because it is
pure. However, if we use Future or Id as the execution context, our application will start listening
to stdin:
val futureEcho: Future[String] = echo[Future]
We have broken purity and are no longer writing FP code: futureEcho is the result of running
echo once. Future conflates the definition of a program with interpreting it (running it). As a
result, applications built with Future are difficult to reason about.
An expression is referentially transparent if it can be replaced with its corresponding value without
changing the program’s behaviour.
Pure functions are referentially transparent, allowing for a great deal of code reuse, performance
optimisation, understanding, and control of a program.
Impure functions are not referentially transparent. We cannot replace echo[Future] with a value,
such as val futureEcho, since the pesky user can type something different the second time.
We can define a simple safe F[_] execution context
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