func circlePerimeter(c Circle) float64 {
	return 2 * math.Pi * c.radius
}
Circle circumference (will call it perimeter for this example)
package main
import (
	"fmt"
	"math"
)
type Circle struct {
	radius float64
}
type Rectangle struct {
	width  float64
	height float64
}
type Triangle struct {
	a float64
	b float64
	c float64
}
func circleArea(c Circle) float64 {
	return math.Pi * math.Pow(c.radius, 2)
}