Implement iteration over GAP iterators (#967)

src/adapter.jl

@@ -485,6 +485,8 @@ function Base.iterate(obj::GapObj)
             # we can still allow iteration util some large bound
             iterate(obj, (1, typemax(Int)))
         end
+    elseif Wrappers.IsIterator(obj)
+        iterate(obj, Wrappers.ShallowCopy(obj))
     elseif Wrappers.IsCollection(obj)
         iterate(obj, Wrappers.Iterator(obj)::GapObj)
     else
@@ -506,6 +508,9 @@ function Base.iterate(obj::GapObj, iter::GapObj)
     end
 end
 
+Base.IteratorEltype(::Type{GapObj}) = Base.EltypeUnknown()
+Base.IteratorSize(::Type{GapObj}) = Base.SizeUnknown()
+
 # copy and deepcopy:
 # The following is just a preliminary solution,
 # in order to avoid Julia crashes when one calls `deepcopy` for a `GapObj`.

src/constructors.jl

@@ -463,7 +463,7 @@ function (::Type{T})(obj::GapObj) where {T<:UnitRange}
         result = obj[1]:obj[len]
     end
 
-    return convert(T, result)
+    return convert(T, result)::T
 end
 
 

src/wrappers.jl

@@ -35,6 +35,7 @@ import GAP: @wrap
 @wrap IsCollection(x::Any)::Bool
 @wrap IsDoneIterator(x::Any)::Bool
 @wrap IsEmpty(x::Any)::Bool
+@wrap IsIterator(x::Any)::Bool
 @wrap IsList(x::Any)::Bool
 @wrap IsMatrixObj(x::Any)::Bool
 @wrap IsPackageLoaded(x::GapObj)::Bool

test/adapter.jl

@@ -1,18 +1,30 @@
 @testset "iteration" begin
+    # iterating over a GAP plain list, which uses index based iteration
     l = GAP.evalstr("[1, 2, 3]")
     lj = collect(l)
-    @test lj isa Vector{Any}
+    @test lj isa Vector{Int}
     @test lj == [1, 2, 3]
     lj = collect(Int, l)
     @test lj isa Vector{Int}
     @test lj == [1, 2, 3]
 
+    # iterating over a GAP object (here, a group) which creates a GAP Iterator object
     s = GAP.Globals.SymmetricGroup(3)
     xs = []
     for x in s
         push!(xs, x)
     end
     @test length(xs) == 6
+
+    # iterating over a GAP iterator
+    gap_iter = GAP.Globals.IteratorOfCombinations(GAP.Obj([1,1,1]))
+    # iterate twice to verify we don't "eat up" the GAP iterator
+    # and don't otherwise change its state
+    for i in 1:2
+        collect(gap_iter)
+        vs = Vector{Int}.(gap_iter)
+        @test vs == [[], [1], [1, 1], [1, 1, 1]]
+    end
 end
 
 @testset "deepcopy" begin