Ensure the single-argument version of (/ x) works.

builtins/builtins.go

@@ -344,6 +344,14 @@ func divideFn(env *env.Environment, args []primitive.Primitive) primitive.Primit
 		return primitive.Error(fmt.Sprintf("argument '%s' was not a number", args[0].ToString()))
 	}
 
+	// If there is only one argument then we return the
+	// reciprocal.
+	//
+	// (i.e. "(/ 3)" == ".3333"
+	if len(args) == 1 {
+		return primitive.Number(1 / v)
+	}
+
 	// now process all the rest of the arguments
 	for _, i := range args[1:] {
 
@@ -642,6 +650,7 @@ func fileWriteFn(env *env.Environment, args []primitive.Primitive) primitive.Pri
 	}
 	return primitive.Nil{}
 }
+
 // gensymFn is the implementation of (gensym ..)
 func gensymFn(env *env.Environment, args []primitive.Primitive) primitive.Primitive {
 	// symbol characters

builtins/builtins_test.go

@@ -558,6 +558,16 @@ func TestDirectoryEntries(t *testing.T) {
 // TestDivide tests "*"
 func TestDivide(t *testing.T) {
 
+	// Test for "equality"
+	//
+	// Because floating points are hard
+	almostEqual := func(a, b float64) bool {
+		// Arbitrary equality threshold
+		float64EqualityThreshold := float64(1.0 / 1000000)
+
+		return math.Abs(a-b) <= float64EqualityThreshold
+	}
+
 	// No arguments
 	out := divideFn(ENV, []primitive.Primitive{})
 
@@ -630,6 +640,35 @@ func TestDivide(t *testing.T) {
 	if n != 4 {
 		t.Fatalf("got wrong result")
 	}
+
+	// Test only a single argument.
+	out = divideFn(ENV, []primitive.Primitive{
+		primitive.Number(12),
+	})
+
+	// Will work
+	n, ok2 = out.(primitive.Number)
+	if !ok2 {
+		t.Fatalf("expected number, got %v", out)
+	}
+	if !almostEqual(float64(n), 0.083333) {
+		t.Fatalf("got wrong result: %f", n)
+	}
+
+	// Another test
+	out = divideFn(ENV, []primitive.Primitive{
+		primitive.Number(-3),
+	})
+
+	// Will work
+	n, ok2 = out.(primitive.Number)
+	if !ok2 {
+		t.Fatalf("expected number, got %v", out)
+	}
+	if !almostEqual(float64(n), -(1 / 3.0)) {
+		t.Fatalf("got wrong result: %f", n)
+	}
+
 }
 
 // TestEnsureHelpPresent ensures that all our built-in functions have
@@ -1288,7 +1327,6 @@ func TestFileWrite(t *testing.T) {
 	// remove the direcotry
 	os.RemoveAll(path)
 
-
 	failure := fileWriteFn(ENV, []primitive.Primitive{
 		primitive.String(target),
 		primitive.String("cake")})