diff --git a/src/Flux.jl b/src/Flux.jl
index d8d20f53..c72b3171 100644
--- a/src/Flux.jl
+++ b/src/Flux.jl
@@ -19,6 +19,7 @@ include("model.jl")
 include("dims/catmat.jl")
 include("dims/batching.jl")
 include("dims/seq.jl")
+include("dims/iter.jl")
 
 include("compiler/code.jl")
 include("compiler/loops.jl")
diff --git a/src/dims/catmat.jl b/src/dims/catmat.jl
index abc241d8..b4c170e2 100644
--- a/src/dims/catmat.jl
+++ b/src/dims/catmat.jl
@@ -15,7 +15,13 @@ size(b::CatMat) = (size(b.data, 1),)
 
 getindex(b::CatMat, i)::eltype(b) = slicedim(b.data, 1, i)
 
-setindex!(b::CatMat, v, i) = b.data[i, :] = v
+setindex!(b::CatMat, v, i::Integer) = b.data[i, :] = v
+
+function setindex!(b::CatMat, xs, ::Colon)
+  for (i, x) in enumerate(xs)
+    b[i] = x
+  end
+end
 
 allequal(xs) = all(x -> x == first(xs), xs)
 
diff --git a/src/dims/iter.jl b/src/dims/iter.jl
new file mode 100644
index 00000000..d35f14d3
--- /dev/null
+++ b/src/dims/iter.jl
@@ -0,0 +1,72 @@
+export Batched
+
+import Base: start, next, done, iteratorsize, iteratoreltype, eltype, length
+
+# Stateful iteration
+
+mutable struct StatefulIter{I,S,T}
+  iter::I
+  state::S
+  next::Nullable{T}
+end
+
+function StatefulIter(itr)
+  state = start(itr)
+  val, state = done(itr, state) ? (Nullable(), state) : next(itr, state)
+  return StatefulIter(itr, state, convert(Nullable, val))
+end
+
+peek(s::StatefulIter) = get(s.next)
+
+function Base.take!(s::StatefulIter)
+  x = peek(s)
+  if !done(s.iter, s.state)
+    s.next, s.state = next(s.iter, s.state)
+  else
+    s.next = Nullable()
+  end
+  return x
+end
+
+Base.isempty(s::StatefulIter) = isnull(s.next)
+Base.eltype(s::StatefulIter) = eltype(s.next)
+
+function taken!(s::StatefulIter, n::Integer)
+  xs = eltype(s)[]
+  for _ = 1:n
+    isempty(s) && break
+    push!(xs, take!(s))
+  end
+  return xs
+end
+
+# Batched
+
+struct Batched{I<:StatefulIter,S}
+  itr::I
+  buf::S
+end
+
+function Batched(itr, n::Integer)
+  n >= 1 || throw(ArgumentError("batch size must be >= 1"))
+  itr = StatefulIter(itr)
+  buf = rebatch(similar(eltype(itr)(), n, size(peek(itr))...))
+  Batched(itr, buf)
+end
+
+iteratoreltype(::Type{<:Batched}) = Base.HasEltype()
+iteratorsize(::Type{<:Batched}) = Base.SizeUnknown()
+
+eltype{T,S}(x::Batched{T,S}) = S
+
+start(::Batched) = ()
+
+next(x::Batched, _) = x.buf, ()
+
+# will be less hacky if https://github.com/JuliaLang/julia/issues/18823
+function done(x::Batched, _)
+  next = taken!(x.itr, length(x.buf))
+  length(next) < length(x.buf) && return true
+  x.buf[:] = next
+  return false
+end