math.go

package math

import (
	"errors"
	m "math"
)

// Finds the minimum value in a slice of numbers
func Min(xs []float64) (float64, error) {
	if len(xs) == 0 {
		return m.NaN(), errors.New("Cannot get minimum value from empty slice")
	}
	min := xs[0]
	for _, v := range xs {
		if v < min {
			min = v
		}
	}
	return min, nil
}

math_test.go

package math

import (
	m "math"
	"testing"
)

type testpair struct {
	values []float64
	min    float64
}

var minTests = []testpair{
	{[]float64{1, 2}, 1},
	{[]float64{1, 1, 1, 1, 1, 1}, 1},
	{[]float64{-1, 1}, -1},
}

var minTestsNaN = []testpair{
	{[]float64{}, m.NaN()},
}

func TestMin(t *testing.T) {
	for _, pair := range minTests {
		if v, err := Min(pair.values); err != nil {
			t.Error("Got unexpected error: ", err)
		} else if v != pair.min {
			t.Error(
				"For", pair.values,
				"expected", pair.min,
				"got", v,
			)
		}
	}
}

func TestMinNaN(t *testing.T) {
	for _, pair := range minTestsNaN {
		v, err := Min(pair.values)
		if err != nil {
			if !m.IsNaN(v) {
				t.Error("Should return NaN")
			}
		} else {
			t.Error("Should've got error for NaN")
		}
	}

}

math.NaN doesn't compare to anything (including itself) so I would separate out the test case and use an error to notify people when they get back a non-float64 return value (which they can check using IsNaN or whatever)