Domain Modeling Made Functional

Documenting Effects

In a previous discussion, we talked about documenting effects in the type signature: What effects could this function have? Could it return an error? Does it do I/O?

Let’s quickly revisit all our dependencies and double-check if we need to be explicit about any effects like this.

Effects in the Validation Step

The validation step has two dependencies: CheckProductCodeExists and CheckAddress Exists.

Let’s start with CheckProductCodeExists:

type CheckProductCodeExists = ProductCode -> bool

Could it return an error, and is it a remote call?

Let’s assume it’s neither of these. Instead, we will expect that a local cached copy of the product catalog is available (remember what Ollie said about autonomy in Chapter 2, ​Understanding ...