# Introduction

The map function is borrowed from functional programming languages in Julia. In many such languages you can safely assume that the functions you pass to map are pure, meaning that they have no side effects and always return the same output for the same passed arguments.

In practice, however, you sometimes pass a non-pure function to map. In this post I present selected cases when this can lead to surprising results.

The post was written under Julia 1.7.0 and PooledArrays.jl 1.4.0.

# Example scenario of non-pure function used with map

Assume you have a vector of data and you want to add some noise to values stored in it. Here is an example how you can do it using map:

julia> using Random

julia> Random.seed!(1234);

julia> x = zeros(5)
5-element Vector{Float64}:
0.0
0.0
0.0
0.0
0.0

julia> map(v -> v + randn(), x)
5-element Vector{Float64}:
-0.3597289068234817
1.0872084924285859
-0.4195896169388487
0.7189099374659392
0.4202471777937789


As expected we have added a different random number to every element of the passed vector.

# Problematic vector types

Let us now move to a case when things get problematic:

julia> using SparseArrays

julia> y = sparse(x)
5-element SparseVector{Float64, Int64} with 0 stored entries

julia> Random.seed!(1234);

julia> map(v -> v + randn(), y)
5-element SparseVector{Float64, Int64} with 5 stored entries:
  =  -0.359729
  =  -0.359729
  =  -0.359729
  =  -0.359729
  =  -0.359729


To our surprise we have added the same noise to every element of y. The reason is that the map function assumes for SparseVector that the function passed to it is pure. Therefore it is called only once for 0.0 element of sparse vector (note that technically this element is not stored in the vector).

Let us try another sparse vector:

julia> z = sparse(ones(5)) .- 1.0
5-element SparseVector{Float64, Int64} with 5 stored entries:
  =  0.0
  =  0.0
  =  0.0
  =  0.0
  =  0.0

julia> Random.seed!(1234);

julia> map(v -> v + randn(), z)
5-element SparseVector{Float64, Int64} with 5 stored entries:
  =  1.08721
  =  -0.41959
  =  0.71891
  =  0.420247
  =  -0.685671


What is going on? There are two things to observe:

1. This time I have stored 0.0 values in the vector so for each entry in the sparse vector I get a new random number generated.
2. The stream of random numbers is shifted by one, as map internally called randn() one more time for the default 0.0 value (which is not stored in this vector in our case).

Things look complex. The short lesson is: do not use the default map with sparse vectors when you want to use a non-pure function.

As a side note, the same problem is present with broadcasting:

julia> Random.seed!(1234);

julia> (v -> v + randn()).(y)
5-element SparseVector{Float64, Int64} with 5 stored entries:
  =  -0.359729
  =  -0.359729
  =  -0.359729
  =  -0.359729
  =  -0.359729


# How to use map with non-pure function and sparse vector

Fortunately there is a relatively simple way to resolve our problems. We need to use the Base.@invoke macro. In this way we can force Julia to use the default map method (not the optimized one that the SparseArrays module defines). Here is how you can do it:

julia> Random.seed!(1234);

julia> Base.@invoke map(v -> v + randn(), y)
5-element Vector{Float64}:
-0.3597289068234817
1.0872084924285859
-0.4195896169388487
0.7189099374659392
0.4202471777937789

julia> Random.seed!(1234);

julia> Base.@invoke map(v -> v + randn(), z)
5-element Vector{Float64}:
-0.3597289068234817
1.0872084924285859
-0.4195896169388487
0.7189099374659392
0.4202471777937789


This time we get the same results as for the x vector. Note that the returned object has Vector type (previously we were getting SparseVector).

# The PooledArrays.jl case

The same problem as for SparseVector is present in PooledArrays.jl. However, here, by default, the map function assumes that the function you pass to it is not-pure for safety:

julia> using PooledArrays

julia> p = PooledArray(x)
5-element PooledVector{Float64, UInt32, Vector{UInt32}}:
0.0
0.0
0.0
0.0
0.0

julia> Random.seed!(1234);

julia> map(v -> v + randn(), p)
5-element PooledVector{Float64, UInt32, Vector{UInt32}}:
-0.3597289068234817
1.0872084924285859
-0.4195896169388487
0.7189099374659392
0.4202471777937789


If you are sure that you are passing a pure function to map in this case you can pass pure=true keyword argument to speed things up:

julia> Random.seed!(1234);

julia> map(v -> v + randn(), p, pure=true)
5-element PooledVector{Float64, UInt32, Vector{UInt32}}:
-0.3597289068234817
-0.3597289068234817
-0.3597289068234817
-0.3597289068234817
-0.3597289068234817


(or to break things, as in this case, since we passed a non-pure function)

# Conclusions

In summary let me highlight that the situation we have discussed in this post is a hard design decision. The reason is that most of the time you indeed pass pure functions to the map function. Therefore having methods that are optimized for performance makes sense. However, sometimes, you want to perform mapping using a non-pure function, in which case you can get surprising results. Hopefully, after reading this post you now know how to handle such cases.

Happy hacking!