Learning Functional Programming in Go

Lineitem slice monoid

Let's see what we can do with a slice of line items.

First, we define an interface that has three methods, Append, Zero, and Reduce. We wrap our line items in the lineitemContainer. Our lineitemContainer is a struct with three fields that correspond to our invoice's line items:

type Lineitem struct {   Quantity   int   Price     int   ListPrice  int}

Our Append method appends the given line item to the slice of line items it's building up that lives in the magical lineitemContainer.

The Zero morphism for a slice is nil.

The src/monoid/lineitem_monoid.go file will have the following code:

package monoidtype LineitemMonoid interface {   Zero() []int   Append(i ...int) LineitemMonoid   Reduce() int}func WrapLineitem(lineitems []Lineitem) ...