3 files changed, 49 insertions, 0 deletions
diff --git a/subex/arithmetic.go b/subex/arithmetic.go
index 7200ac7..5497a68 100644
--- a/subex/arithmetic.go
+++ b/subex/arithmetic.go
@@ -85,3 +85,36 @@ func multiplyValues(atoms []walk.Atom) ([]walk.Atom, error) {
 		return []walk.Atom{walk.ValueNumber(product)}, nil
 	}
 }
+
+// Does tries to cast all to numbers and negates them
+func negateValues(atoms []walk.Atom) ([]walk.Atom, error) {
+	var negatedNumbers []walk.Atom
+	values, err := walk.MemoryCompound(atoms)
+	if err != nil {
+		return nil, err
+	}
+	for _, value := range values {
+		switch v := value.(type) {
+			case walk.ValueNull:
+				negatedNumbers = append(negatedNumbers, walk.ValueNumber(0))
+			case walk.ValueBool:
+				if bool(v) {
+					negatedNumbers = append(negatedNumbers, walk.ValueNumber(-1))
+				} else {
+					negatedNumbers = append(negatedNumbers, walk.ValueNumber(0))
+				}
+			case walk.ValueNumber:
+				negatedNumbers = append(negatedNumbers, walk.ValueNumber(-v))
+			case walk.ValueString:
+				num, err := strconv.ParseFloat(string(v), 64)
+				if err == nil {
+					negatedNumbers = append(negatedNumbers, walk.ValueNumber(-num))
+				} else {
+					return nil, errors.New("Tried to sum non-castable string")
+				}
+			default:
+				return nil, errors.New("Tried to sum non-number")
+		}
+	}
+	return negatedNumbers, nil
+}
diff --git a/subex/parse.go b/subex/parse.go
index f73d060..8635186 100644
--- a/subex/parse.go
+++ b/subex/parse.go
@@ -238,6 +238,8 @@ func parseSubex(l *RuneReader, minPower int) SubexAST {
 				lhs = SubexASTSum {lhs}
 			case r == '*' && minPower <= 8:
 				lhs = SubexASTProduct {lhs}
+			case r == '-' && minPower <= 8:
+				lhs = SubexASTNegate {lhs}
 			case r == '$' && minPower <= 8:
 				slot := l.next()
 				if slot == eof {
diff --git a/subex/subexast.go b/subex/subexast.go
index abf0ca6..3c807ee 100644
--- a/subex/subexast.go
+++ b/subex/subexast.go
@@ -209,3 +209,17 @@ func (ast SubexASTProduct) compileWith(next SubexState) SubexState {
 		}),
 	}
 }
+
+// Runs the content Subex, if all outputted atoms can be cast to numbers, outputs them all negated
+// Rejects if this fails
+type SubexASTNegate struct {
+	content SubexAST
+}
+func (ast SubexASTNegate) compileWith(next SubexState) SubexState {
+	return &SubexCaptureBeginState {
+		next: ast.content.compileWith(&SubexArithmeticEndState {
+			next: next,
+			calculate: negateValues,
+		}),
+	}
+}