Implement AbsoluteValue function in stdlib

src/stdlib/functions/AbsoluteValue.cs

@@ -13,15 +13,46 @@ namespace stdlib.functions
         {
             get
             {
-                yield return new FunctionArgument() { };
+                yield return new FunctionArgument(
+                    name: "value",
+                    fv: new FunctionValue(ValueType.Numeric, 0m)
+                );
             }
         }
 
-        public FunctionValue ReturnValue => throw new NotImplementedException();
+        public FunctionValue ReturnValue => new FunctionValue(ValueType.Numeric, 0m);
 
         public FunctionValue Execute(params FunctionArgument[] args)
         {
-            throw new NotImplementedException();
+            if (args == null || args.Length < 1 || args[0] == null || args[0].Value == null)
+            {
+                return new FunctionValue(ValueType.None, null);
+            }
+
+            try
+            {
+                var input = args[0].Value;
+                // Try to interpret both numeric and numeric-like string inputs
+                decimal number;
+                if (input.Type == ValueType.Numeric)
+                {
+                    number = Convert.ToDecimal(input.Value);
+                }
+                else if (input.Type == ValueType.String && input.Value is string s)
+                {
+                    number = Convert.ToDecimal(s);
+                }
+                else
+                {
+                    return new FunctionValue(ValueType.None, null);
+                }
+
+                return new FunctionValue(ValueType.Numeric, Math.Abs(number));
+            }
+            catch
+            {
+                return new FunctionValue(ValueType.None, null);
+            }
         }
     }
 }