Add support for negative variables (unary minus) (#35)

src/evaluator.jl

@@ -19,6 +19,7 @@ function eval_AST(expr::BaseModelicaExpr)
             BaseModelicaFactor(base, exp) => (f(base))^f(exp)
             BaseModelicaSum(left, right) => (f(left)) + (f(right))
             BaseModelicaMinus(left, right) => f(left) - f(right)
+            BaseModelicaUnaryMinus(operand) => -f(operand)
             BaseModelicaProd(left, right) => f(left) * f(right)
             BaseModelicaDivide(left, right) => f(left) / f(right)
             BaseModelicaNot(relation) => !(f(relation))

src/parser.jl

@@ -63,6 +63,7 @@ end
     BaseModelicaIdentifier(name)
     BaseModelicaSum(left, right)
     BaseModelicaMinus(left, right)
+    BaseModelicaUnaryMinus(operand)
     BaseModelicaProd(left, right)
     BaseModelicaFactor(base, exp)
     BaseModelicaElementWiseFactor(base, exp)
@@ -126,6 +127,11 @@ function create_term(input_list)
 end
 
 function create_arithmetic_expression(input_list)
+    # Handle unary minus case - if first element is a minus operator
+    if length(input_list) >= 2 && input_list[1] isa BMSubtract
+        return BaseModelicaUnaryMinus(input_list[2])
+    end
+
     left_el = input_list[1]
     for (i, element) in enumerate(input_list)
         left_el = @match element begin

test/runtests.jl

@@ -19,6 +19,11 @@ if GROUP == "All" || GROUP == "Core"
             arith_test = only(PC.parse_one("5 + 6*(45 + 9^2)^2", BM.arithmetic_expression))
             @test arith_test isa BM.BaseModelicaSum
             @test BM.eval_AST(arith_test) == 95261.0
+
+            # Test unary minus parsing (issue #35)
+            unary_minus_test = only(PC.parse_one("-5", BM.arithmetic_expression))
+            @test unary_minus_test isa BM.BaseModelicaUnaryMinus
+            @test BM.eval_AST(unary_minus_test) == -5.0
 
             newton_path = joinpath(
                 dirname(dirname(pathof(BM))), "test", "testfiles", "NewtonCoolingBase.mo")
@@ -27,6 +32,15 @@ if GROUP == "All" || GROUP == "Core"
             newton_system = BM.baseModelica_to_ModelingToolkit(newton_cooling)
             @test newton_system isa ODESystem
             @test parse_basemodelica("testfiles/NewtonCoolingBase.mo") isa ODESystem
+
+            # Test parsing with negative variables (issue #35)
+            negative_path = joinpath(
+                dirname(dirname(pathof(BM))), "test", "testfiles", "NegativeVariable.mo")
+            negative_package = BM.parse_file(negative_path)
+            @test negative_package isa BM.BaseModelicaPackage
+            negative_system = BM.baseModelica_to_ModelingToolkit(negative_package)
+            @test negative_system isa ODESystem
+            @test parse_basemodelica("testfiles/NegativeVariable.mo") isa ODESystem
         end
     end
 end

test/testfiles/NegativeVariable.mo

@@ -0,0 +1,7 @@
+package 'Negate'
+  model 'Negate'
+    Real 'x';
+  equation
+    der('x') = -'x';
+  end 'Negate';
+end 'Negate';