Effects, functionally

What are functional effects, really?

Functional effects can be used to model side effects in a purely functional program. When we think about what side effects are in a non-pure program we can say that they are all the other things that can happen besides passing around a value. Functional effects are a functional way to keep this separation of the effect and passing around a value.

Functional effects consist of:

• a polymorphic data type that separates the value from the effect
• combinators on that data type that allow you to work with the value without having to deal with the effect

Let’s look at some examples (from PureScript):

Partiality

data Maybe a = Nothing | Just a

• Separates the value a from the effect of partiality enabled by the Nothing constructor
• Effect: partiality, being able to short circuit a computation, nondeterministic choice
• Useful combinators: bind, apply, alt

Read-only context

data Reader r a = Reader (r -> a)

• Separates the value a from the effect of reading a value r from the context
• Effect: access to a value inside the computation, a “global value” so to speak
• Useful combinators: bind, apply, ask, local

Read-write context

data State s a = State (s -> Tuple a s)

• Separates the value a from the effect of state s
• Effect: access to a read-write context within the computation
• Useful combinators: bind, apply, put, modify

No effects

data Identity a = Identity a

• Separates the value a from the “null” effect. A surprisingly useful data type that can be used in places where no effects are needed.
• Effect: none
• Useful combinators: bind, apply, alt, and many others