LazyBroadcast.jl

LazyBroadcast.jl provides a function, lazy_broadcast, and macro @lazy_broadcast, to transform a given Julia broadcast expression into a Base.Broadcast.Broadcasted object, without materializing it.

For more information about Julia broadcasting, please see https://docs.julialang.org/en/v1/manual/arrays/#Broadcasting.

This utility is useful in a few situations:

• Debugging broadcast machinery
• Fusing operations in multiple broadcast expressions (alternatively, see MultiBroadcastFusion.jl)
• Delaying execution of a broadcast expression

For not-in-place expressions, lazy_broadcast/@lazy_broadcast simply returns the instantiated broadcasted object, via Base.Broadcast.instantiate (Base.Broadcast.broadcasted(x)), of the right-hand-side:

using Test
import LazyBroadcast: @lazy_broadcast, lazy_broadcast
import Base.Broadcast: instantiate, broadcasted, materialize

a = rand(3,3)
b = rand(3,3)

@testset "lazy_broadcast" begin
    bc = lazy_broadcast.(a .+ b) # get the broadcasted object
    @test instantiate(broadcasted(+, a, b)) == bc
    @test materialize(bc) == @. a + b # materialize the broadcasted object
end

@testset "@lazy_broadcast" begin
    bc = @lazy_broadcast @. a + b # get the broadcasted object
    @test instantiate(broadcasted(+, a, b)) == bc
    @test materialize(bc) == @. a + b # materialize the broadcasted object
end

lazy_broadcast does not support in-place expressions (as is supported in MultiBroadcastFusion.jl).

Acknowledgement

The original implementation of LazyBroadcast involved a similar recipe to MultiBroadcastFusion, which has different yet justified needs for its implementation. Since then, DontMaterialize.jl was developed, which satisfied most needs by LazyBroadcast using a significantly more elegant approach, discussed here. So, we've borrowed that implementation in order to be more consistent with Base.Broadcast semantics, and provide users with less surprising behavior. We've also kept code_lowered_single_expression, as this is one feature that DontMaterialize.jl does not offer (transforming broadcasted Expr to Expr).