Add tests for negative degree (#271)

* Add tests for negative degree

* Fix format

src/monomial_vector.jl

@@ -66,7 +66,7 @@ end
 Returns the return type of `monomial_vector`.
 """
 function monomial_vector_type(
-    X::Union{AbstractVector{PT},Type{<:AbstractVector{PT}}},
+    ::Union{AbstractVector{PT},Type{<:AbstractVector{PT}}},
 ) where {PT<:_APL}
     return monomial_vector_type(PT)
 end
@@ -109,3 +109,13 @@ Returns the vector of monomials in the entries of `X` in increasing order and wi
 Calling `merge_monomial_vectors` on ``[[xy, x, xy], [x^2y, x]]`` should return ``[x^2y, xy, x]``.
 """
 merge_monomial_vectors(X) = monomial_vector(reduce(vcat, X))
+
+function error_for_negative_degree(deg)
+    if deg < 0
+        throw(
+            ArgumentError(
+                "The degree should be a nonnegative number but the provided degree `$deg` is negative.",
+            ),
+        )
+    end
+end

test/monomial_vector.jl

@@ -100,4 +100,11 @@
             v[1]^3,
         ]
     end
+
+    @testset "Negative degree" begin
+        Mod.@polyvar x[1:3]
+        @test_throws ArgumentError monomials(x, -1)
+        @test_throws ArgumentError monomials(x, -1:1)
+        @test_throws ArgumentError monomials(x, [1, -1])
+    end
 end

test/ncmonomial.jl

@@ -37,7 +37,10 @@
     end
 end
 @testset "Non-commutative MonomialVector" begin
-    Mod.@ncpolyvar x y
+    v = Mod.@ncpolyvar x y
+    @test_throws ArgumentError monomials(v, -1)
+    @test_throws ArgumentError monomials(v, -1:1)
+    @test_throws ArgumentError monomials(v, [1, -1])
     X = empty_monomial_vector(typeof(x))
     @test iszero(nvariables(X))
     @test isempty(variables(X))