Scheme primitives

primitives.go

package primitives

import (
	"errors"
	"reflect"
)

// The definitions in functions are from THE book:
// Friedman & Felleisen. The Little Schemer, Fourth Edition, MIT Press, 1996.

// The Law of Car:
// The primitive *car* is defined only for non-empty lists.
// (p. 5)
func car(v interface{}) (interface{}, error) {
	wrapper := reflect.ValueOf(v)
	// v is not a list
	if !(wrapper.Kind() == reflect.Array || wrapper.Kind() == reflect.Slice) {
		return nil, errors.New("Not a list")
	}

	// wrapper is an empty list
	if wrapper.Len() == 0 {
		return nil, errors.New("Empty list")
	}

	// Returns the first element of the list, indicating no error
	return wrapper.Index(0), nil
}

primitives_test.go

package primitives

import "testing"

func TestCar(t *testing.T) {
	var tests = []struct {
		input []int
		want  int
	}{
		{[]int{10, 20}, 10},
		{[]int{30}, 30},
	}

	for _, test := range tests {
		if got, _ := car(test.input); got != test.want {
			t.Errorf("car(%v)=%v, got %v", test.input, test.want, got)
		}
	}
}