# ANN: Transducers.jl, efficient and composable algorithms for map- and reduce-like operations

**URL:** <https://discourse.julialang.org/t/ann-transducers-jl-efficient-and-composable-algorithms-for-map-and-reduce-like-operations/19159>\
**Category:** Package Announcements\
**Created:** [January 1, 2019, 3:30am UTC](https://discourse.julialang.org/t/ann-transducers-jl-efficient-and-composable-algorithms-for-map-and-reduce-like-operations/19159 "2019-01-01T03:30:43Z")\
**Posts on this page:** 1\
**Showing post:** 7

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**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [January 1, 2019, 9:44pm UTC](https://discourse.julialang.org/t/ann-transducers-jl-efficient-and-composable-algorithms-for-map-and-reduce-like-operations/19159/7 "2019-01-01T21:44:54Z")

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Good point. Here is an example that is very “hostile” to iterator:

```julia
julia> xs = 1:2^15
1:32768

julia> @btime foldl(+, Iterators.flatten(1:x for x in xs))
  200.448 ms (10 allocations: 352 bytes)
5864598896640

julia> @btime mapfoldl(Map(x -> 1:x) |> Cat(), +, xs)
  45.532 μs (18 allocations: 880 bytes)
5864598896640

```

Piggybacking on [Skipnothing is missing](https://discourse.julialang.org/t/skipnothing-is-missing/19155), here is a more reasonable case for comparison:

```julia
julia> xs = [abs(x) > 1 ? nothing : x for x in randn(2^20)];

julia> @btime foldl(+, Iterators.filter(!isnothing, xs); init=0.0)
  4.084 ms (4 allocations: 80 bytes)
779.0559461186579

julia> @btime mapfoldl(Filter(!isnothing), +, xs, init=0.0)
  2.913 ms (5 allocations: 80 bytes)
779.0559461186579

```

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