1 files changed, 12 insertions, 8 deletions
diff --git a/const.go b/const.go
index ff0d09c..3808e82 100644
--- a/const.go
+++ b/const.go
@@ -2,16 +2,20 @@ package main
 
 import "math"
 
-// parseConstant returns nil if s is not a valid constant.
-func parseConstant(s string) *float64 {
+func parseConstant(s string) (float64, error) {
 	switch s {
 	case "pi":
-		// Assign to variable because can't take address of constant.
-		var pi float64 = math.Pi
-		return &pi
+		return math.Pi, nil
 	case "e":
-		var e float64 = math.E
-		return &e
+		return math.E, nil
 	}
-	return nil
+	return 0, InvalidConstantErr{s}
+}
+
+type InvalidConstantErr struct {
+	s string
+}
+
+func (e InvalidConstantErr) Error() string {
+	return "invalid constant: " + e.s
 }