fix: negated number left as the last expression would fail

mathexpr.cabal

@@ -1,5 +1,5 @@
 name:                mathexpr
-version:             0.2.0.0
+version:             0.2.0.1
 synopsis:            Parse and evaluate math expressions with variables and functions
 description:         A simple tool to evaluate math expressions as strings with support for custom functions and operators
 homepage:            https://github.com/mdibaiee/mathexpr

src/Data/MathExpr.hs

@@ -80,6 +80,7 @@ module Data.MathExpr
           helper :: [String] -> [String] -> Double
           -- negative numbers come in the form ["num", "-"]
           helper [] [o] = read o
+          helper [n, "-"] [] = negate . read $ n
           helper (c:cs) os
             | c == "-" && length os < 2 =
                 let result :: Double = negate . read . head $ cs