diff --git a/mshell/ArityCheck.go b/mshell/ArityCheck.go
new file mode 100644
index 00000000..3d62d76b
--- /dev/null
+++ b/mshell/ArityCheck.go
@@ -0,0 +1,84 @@
+package main
+
+
+type ArityCheckResult struct {
+	InputCount int
+	OutputCount int
+}
+
+type ArityTracking struct {
+	CurrentSize int
+	MinSize int // Expected to be <= 0
+}
+
+func (a *ArityTracking) Apply(input int, output int) {
+	// Assert that input and output are non-negative
+	if input < 0 || output < 0 {
+		panic("Apply called with negative input or output")
+	}
+
+	if a.CurrentSize - input < a.MinSize {
+		a.MinSize = a.CurrentSize - input
+	}
+
+	a.CurrentSize += output - input
+}
+
+func (a *ArityTracking) Result() ArityCheckResult {
+	return ArityCheckResult{
+		InputCount:  -a.MinSize,
+		OutputCount: a.CurrentSize,
+	}
+}
+
+
+func (a *ArityTracking) Decrement(num int) {
+	// Assert that num is > 0
+	if num <= 0 {
+		panic("Decrement called with non-positive number")
+	}
+	a.CurrentSize -= num
+	if a.CurrentSize < a.MinSize {
+		a.MinSize = a.CurrentSize
+	}
+}
+
+func ArityCheck(objects []MShellParseItem, definitions []MShellDefinition) ArityCheckResult {
+	index := 0
+
+	arity := ArityTracking {
+		CurrentSize: 0,
+		MinSize: 0,
+	}
+
+MainLoop:
+	for index < len(objects) {
+		t := objects[index]
+		index++
+
+		switch t := t.(type) {
+		case *MShellParseList:
+		case *MShellParseDict:
+		case *MShellParseQuote:
+			arity.Apply(0, 1)
+		case *MShellIndexerList:
+			arity.Apply(1, 1)
+		case MShellVarstoreList:
+			arity.Apply(len(t.VarStores), 0)
+		case Token:
+			if t.Type == EOF {
+				return arity.Result()
+			} else if t.Type == LITERAL {
+				// Check for definitions
+				for _, definition := range definitions {
+					if definition.Name == t.Lexeme {
+						arity.Apply(len(definition.TypeDef.InputTypes), len(definition.TypeDef.OutputTypes))
+						continue MainLoop
+					}
+				}
+			}
+		}
+	}
+
+	return arity.Result()
+}