Implement monomial constructor (#94)

Author: blegat

src/types.jl

@@ -43,6 +43,10 @@ MP.nvariables(::Type{<:Monomial{V}}) where {V} = length(V)
 MP.nvariables(m::Monomial) = nvariables(typeof(m))
 MP.monomial_type(::Type{V}) where V<:Variable = monomial_type(V())
 MP.monomial_type(v::Variable) = Monomial{(v,), 1}
+function MP.monomial(vars::Tuple, exps::NTuple{N,Int}) where {N}
+    @assert length(vars) == length(exps)
+    return Monomial{vars,N}(exps)
+end
 
 MP.exponents(m::Monomial) = m.exponents
 MP.exponent(m::Monomial, i::Integer) = m.exponents[i]

test/polynomials.jl

@@ -97,6 +97,10 @@ end
         @test @inferred(Monomial{tuple(), 0}()) == @inferred(x^0)
 
         @test @inferred(Monomial{(x, y)}([1, 2])) == Monomial{(x, y)}((1, 2))
+        @test @inferred(monomial((x, y), (1, 2))) == Monomial{(x, y)}((1, 2))
+        @test_throws AssertionError monomial((x,), (1, 2))
+        @test_throws AssertionError monomial((y, x), (1, 2))
+        @test_throws AssertionError monomial((x, x), (1, 2))
     end
 
     @testset "terms" begin