# Mapslices() warntype?

**URL:** <https://discourse.julialang.org/t/mapslices-warntype/36844>\
**Category:** New to Julia\
**Tags:** mapslices\
**Created:** [April 1, 2020, 11:20am UTC](https://discourse.julialang.org/t/mapslices-warntype/36844 "2020-04-01T11:20:51Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![tomtom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomtom/32/5106_2.png) [@tomtom](https://discourse.julialang.org/u/tomtom)\
**Post date:** [April 1, 2020, 11:20am UTC](https://discourse.julialang.org/t/mapslices-warntype/36844/1 "2020-04-01T11:20:51Z")

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I’m wondered seeing that `mapslices()` gives warntype as below:

```julia
julia> const mat = randn(3,2)
3×2 Array{Float64,2}:
  1.49498 -1.98461  
 -0.935659 -0.0317199
  0.783414 -0.284129 

julia> mapslices(prod, mat, dims = 1)
1×2 Array{Float64,2}:
 -1.09583 -0.0178864

julia> @code_warntype mapslices(prod, mat, dims = 1)
Body::Any
...
5 ┄ %16 = Base.:(var"#mapslices#109")(dims, @_3, f, A)::Any
└── return %16

```

is there any way to avoid the warntype? thanks.

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**Author:** ![mschauer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mschauer/32/13946_2.png) [@mschauer](https://discourse.julialang.org/u/mschauer)\
**Post date:** [April 1, 2020, 11:40am UTC](https://discourse.julialang.org/t/mapslices-warntype/36844/2 "2020-04-01T11:40:15Z")

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Mapslices can’t be inferred, because Julia has no way of telling what `prod` applied to a slice `mat` looks like without trying. Therefore `mapslices` first figures that out by calling `prod` on the first slice and then it calls an inner function which then is “typestable”. [[source]](https://github.com/JuliaLang/julia/blob/afb9ad4033bc3ebf8310228ac48c348aafc11dc1/base/abstractarray.jl#L2038)

See keyword “function barrier” in the [[performance tips]](https://docs.julialang.org/en/v1/manual/performance-tips/#kernel-functions-1). Therefore you do not need to worry much about that warning, are you calling `mapslices` in highly performance relevant code or inner loops?

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**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [April 1, 2020, 12:00pm UTC](https://discourse.julialang.org/t/mapslices-warntype/36844/3 "2020-04-01T12:00:17Z")

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`mapslices` just doesn’t behave very well, IMO, and you are much better off building the same thing from other pieces. These are fine:

```julia
@code_warntype map(prod, eachcol(mat))
@code_warntype map(prod, eachslice(mat, dims=2)) # same, with keyword

```

although of course you can write `prod(mat, dims=1)`, but I guess it’s an example. If your function returns arrays, you want something like this:

```julia
f(x) = vcat(x, x .^ 2)
mapslices(f, mat, dims=1) == reduce(hcat, map(f, eachcol(mat))) # true
@code_warntype mapslices(f, mat, dims=1) # Any
@code_warntype reduce(hcat, map(f, eachcol(mat))) # Array{Float64,2}

```

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<div class="post-metadata">

**Author:** ![tomtom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomtom/32/5106_2.png) [@tomtom](https://discourse.julialang.org/u/tomtom)\
**Post date:** [April 1, 2020, 1:15pm UTC](https://discourse.julialang.org/t/mapslices-warntype/36844/4 "2020-04-01T13:15:31Z")

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> [@mschauer](#):
>
> See keyword “function barrier” in the [[performance tips]](https://docs.julialang.org/en/v1/manual/performance-tips/#kernel-functions-1). Therefore you do not need to worry much about that warning, are you calling `mapslices` in highly performance relevant code or inner loops?

yes, using in performance sensitive inner loop… so, even with the function barrier `inner_mapslices!()`, seems like `mapslices()` is still very slow compared to `map( , eachcol() )` as suggested by @mcabbott:

```julia
julia> @btime prod($mat, dims = $1);
  57.745 ns (1 allocation: 96 bytes)

julia> @btime map($prod, eachcol($mat));
  76.787 ns (6 allocations: 256 bytes)

julia> @btime map($prod, eachslice($mat, dims = $2));
  585.473 ns (7 allocations: 272 bytes)

julia> @btime mapslices($prod, $mat, dims = $1);
  4.639 μs (54 allocations: 2.56 KiB)

```

it’s interesting to see that `map( , eachslice())` is that much slower than `map(, eachcol() )` !

another thing to note is the confusion of `dims`: `1` is used in `prod()` and `mapslices()` but `2` should be used in `eachslice()` !!!

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<div class="post-metadata">

**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [April 1, 2020, 1:24pm UTC](https://discourse.julialang.org/t/mapslices-warntype/36844/5 "2020-04-01T13:24:55Z")

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> [@tomtom](#):
>
> `dims` : `1` is used in `prod()` and `mapslices()` but `2` should be used in `eachslice()`

Yes, see e.g. [mapslices, map interpret dims differently · Issue #34840 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/34840), [Equivalent of mapslices with views? · Issue #29146 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/29146), and maybe [https://discourse.julialang.org/t/ann-lazystack-slicemap-starslice](https://discourse.julialang.org/t/ann-lazystack-slicemap-starslice) .