Error handling for repeated variable names

src/macros.jl

@@ -251,9 +251,11 @@ function parse_system(exprs)
     constraints = Vector{Expr}()
 
     # define defaults for state, noise and input symbol and default dimension
-    state_var = :x
-    input_var = :u
-    noise_var = :w
+    default_input_var = :u
+    default_noise_var = :w
+    state_var = nothing
+    input_var = nothing
+    noise_var = nothing
     dimension = nothing
     initial_state = nothing # for initial-value problems
 
@@ -323,6 +325,26 @@ function parse_system(exprs)
     nsets = length(constraints)
     nsets > 3 && throw(ArgumentError("cannot parse $nsets set constraints"))
 
+    # error handling for variable names
+    state_var == nothing && throw(ArgumentError("the state variable was not found"))
+    got_input_var = input_var != nothing
+    got_noise_var = noise_var != nothing
+    if got_input_var && (state_var == input_var)
+         throw(ArgumentError("state and input variables have the same name `$(state_var)`"))
+    elseif got_noise_var && (state_var == noise_var)
+         throw(ArgumentError("state and noise variables have the same name `$(state_var)`"))
+    elseif got_input_var && got_noise_var && (input_var == noise_var)
+         throw(ArgumentError("input and noise variables have the same name `$(input_var)`"))
+    end
+
+    # assign default values
+    if !got_input_var
+        input_var = default_input_var
+    end
+    if !got_noise_var
+        noise_var = default_noise_var
+    end
+
     return dynamic_equation, AT, constraints,
            state_var, input_var, noise_var, dimension, initial_state
 end

test/@system.jl

@@ -234,6 +234,9 @@ end
 
     sys = @system(x⁺ = Ax + Bu + c + Dw, x ∈ X, u ∈ U1, w ∈ W1, w:noise, u:input)
     @test sys == NoisyConstrainedAffineControlDiscreteSystem(A, B, c, D, X, U1, W1)
+
+    # check error handling for repeated variable names
+    @test_throws ArgumentError @system(x⁺ = Ax + Bw + Dw, x ∈ X, w ∈ U1, w ∈ W1, noise=w, input=w)
 end
 
 @testset "@system for black-box discrete systems" begin