Make multiexponents type stable (#136)

* added `Base.eltype()` method for `MultiExponents`

* added tests for type-stability

* Update test/multinomials.jl

* Update test/multinomials.jl

---------

Co-authored-by: Jack Shannon <j.b.shannon@lse.ac.uk>
Co-authored-by: Chengyu Han <cyhan.dev@outlook.com>

Author: 3 people

src/multinomials.jl

@@ -26,6 +26,7 @@ function Base.iterate(m::MultiExponents, s = nothing)
 end
 
 Base.length(m::MultiExponents) = length(m.c)
+Base.eltype(::Type{MultiExponents{T}}) where {T} = Vector{Int}
 
 """
     multiexponents(m, n)
@@ -35,15 +36,16 @@ Returns the exponents in the multinomial expansion (x₁ + x₂ + ... + xₘ)ⁿ
 For example, the expansion (x₁ + x₂ + x₃)² = x₁² + x₁x₂ + x₁x₃ + ...
 has the exponents:
 
-    julia> collect(multiexponents(3, 2))
-
-    6-element Array{Any,1}:
-     [2, 0, 0]
-     [1, 1, 0]
-     [1, 0, 1]
-     [0, 2, 0]
-     [0, 1, 1]
-     [0, 0, 2]
+```julia-repl
+julia> collect(multiexponents(3, 2))
+6-element Vector{Vector{Int64}}:
+ [2, 0, 0]
+ [1, 1, 0]
+ [1, 0, 1]
+ [0, 2, 0]
+ [0, 1, 1]
+ [0, 0, 2]
+```
 """
 function multiexponents(m, n)
     # number of stars and bars = m+n-1

test/multinomials.jl

@@ -25,4 +25,10 @@ using LinearAlgebra
     @test [multiexponents(2, 2)...] == [[2, 0], [1, 1], [0, 2]]
     @test [multiexponents(3, 3)...] == [[3, 0, 0], [2, 1, 0], [2, 0, 1], [1, 2, 0], [1, 1, 1], [1, 0, 2], [0, 3, 0], [0, 2, 1], [0, 1, 2], [0, 0, 3]]
 
+    @test length(multiexponents(1, 1)) == 1
+    @test length(multiexponents(2, 2)) == 3
+
+    # type-stability
+    @test typeof([multiexponents(1, 1)...]) == Vector{Vector{Int}}
+    @test eltype(multiexponents(1, 1)) == Vector{Int}
 end