RFC: Modified map for different length iterators

base/iterators.jl

@@ -264,6 +264,33 @@ struct Zip{Is<:Tuple}
     is::Is
 end
 
+"""
+    truncate(s1, s2)
+
+Check two array shapes for compatibility, and return whichever shape has less dimensions.
+
+# Examples
+```jldoctest
+julia> Iterators.truncate([1,2,3], 1:10)
+(Base.OneTo(3),)
+
+julia> Iterators.truncate([2,3,1], [2,3,5,6,4])
+(Base.OneTo(3),)
+```
+"""
+
+function truncate(a::AbstractArray, b::AbstractArray)
+    truncate(axes(a), axes(b))
+end
+
+function truncate(a::Tuple{Base.OneTo{Int}}, b::Tuple{Base.OneTo{Int}})
+    if length(a[1]) < length(b[1])
+        return a
+    else
+        return b
+    end
+end
+
 """
     zip(iters...)
 
@@ -313,7 +340,9 @@ function _zip_min_length(is)
 end
 _zip_min_length(is::Tuple{}) = nothing
 size(z::Zip) = _promote_shape(map(size, z.is)...)
-axes(z::Zip) = _promote_shape(map(axes, z.is)...)
+axes(z::Zip) = truncate(map(axes, z.is)...)
+truncate(p, q...) = truncate(p, truncate(q...))
+truncate(p) = p
 _promote_shape(a, b...) = promote_shape(a, _promote_shape(b...))
 _promote_shape(a) = a
 eltype(::Type{Zip{Is}}) where {Is<:Tuple} = _zip_eltype(Is)

test/iterators.jl

@@ -76,6 +76,13 @@ end
 
 @test_throws MethodError collect(rest(countfrom(1), 5))
 
+# truncate
+# --------
+@test Iterators.truncate([1,2,3], [2,3,4,5]) == (Base.OneTo(3),)
+@test Iterators.truncate(1:10, 1:50) == (Base.OneTo(10),)
+@test Iterators.truncate([1,2,3], 1:10) == (Base.OneTo(3),)
+@test Iterators.truncate((Base.OneTo(3),), (Base.OneTo(5),)) == (Base.OneTo(3),)
+
 # countfrom
 # ---------
 let i = 0, k = 1