add Flatten iterator

base/iterator.jl

@@ -341,3 +341,57 @@ function collect{I<:IteratorND}(g::Generator{I})
     dest[1] = first
     return map_to!(g.f, 2, st, dest, g.iter)
 end
+
+# flatten an iterator of iterators
+
+immutable Flatten{I}
+    it::I
+end
+
+"""
+    flatten(iter)
+
+Given an iterator that yields iterators, return an iterator that yields the
+elements of those iterators.
+Put differently, the elements of the argument iterator are concatenated. Example:
+
+    julia> collect(flatten((1:2, 8:9)))
+    4-element Array{Int64,1}:
+     1
+     2
+     8
+     9
+"""
+flatten(itr) = Flatten(itr)
+
+eltype{I}(::Type{Flatten{I}}) = eltype(eltype(I))
+
+function start(f::Flatten)
+    local inner, s2
+    s = start(f.it)
+    d = done(f.it, s)
+    # this is a simple way to make this function type stable
+    d && throw(ArgumentError("argument to Flatten must contain at least one iterator"))
+    while !d
+        inner, s = next(f.it, s)
+        s2 = start(inner)
+        !done(inner, s2) && break
+        d = done(f.it, s)
+    end
+    return s, inner, s2
+end
+
+function next(f::Flatten, state)
+    s, inner, s2 = state
+    val, s2 = next(inner, s2)
+    while done(inner, s2) && !done(f.it, s)
+        inner, s = next(f.it, s)
+        s2 = start(inner)
+    end
+    return val, (s, inner, s2)
+end
+
+@inline function done(f::Flatten, state)
+    s, inner, s2 = state
+    return done(f.it, s) && done(inner, s2)
+end

test/functional.jl

@@ -166,6 +166,19 @@ end
 @test isempty(collect(Base.product(1:0,1:2)))
 @test length(Base.product(1:2,1:10,4:6)) == 60
 
+# flatten
+# -------
+
+import Base.flatten
+
+@test collect(flatten(Any[1:2, 4:5])) == Any[1,2,4,5]
+@test collect(flatten(Any[flatten(Any[1:2, 6:5]), flatten(Any[10:7, 10:9])])) == Any[1,2]
+@test collect(flatten(Any[flatten(Any[1:2, 4:5]), flatten(Any[6:7, 8:9])])) == Any[1,2,4,5,6,7,8,9]
+@test collect(flatten(Any[flatten(Any[1:2, 6:5]), flatten(Any[6:7, 8:9])])) == Any[1,2,6,7,8,9]
+@test collect(flatten(Any[2:1])) == Any[]
+@test eltype(flatten(UnitRange{Int8}[1:2, 3:4])) == Int8
+@test_throws ArgumentError collect(flatten(Any[]))
+
 # foreach
 let
     a = []