# Preventing broadcast from creating anonymous functions on lowering for a custom type

**URL:** <https://discourse.julialang.org/t/preventing-broadcast-from-creating-anonymous-functions-on-lowering-for-a-custom-type/4508>\
**Category:** General Usage\
**Created:** [June 28, 2017, 3:07am UTC](https://discourse.julialang.org/t/preventing-broadcast-from-creating-anonymous-functions-on-lowering-for-a-custom-type/4508 "2017-06-28T03:07:03Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![fjarri](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fjarri/32/630_2.png) [@fjarri](https://discourse.julialang.org/u/fjarri)\
**Post date:** [June 28, 2017, 3:07am UTC](https://discourse.julialang.org/t/preventing-broadcast-from-creating-anonymous-functions-on-lowering-for-a-custom-type/4508/1 "2017-06-28T03:07:03Z")

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I want to implement my own type that behaves similarly to an `Array`, in particular participates in the same arithmetic expressions. Except, instead of eager evaluation, the arithmetic operations should just return an expression tree. Since, for example, `.*` or `.^` are different semantically from `*` and `^` (i.e. it’s not just a performance optimization), the difference between dotted and non-dotted operators must be preserved in the tree.

In Julia 0.5 it was pretty straightforward: you would just add methods to `*` and `.*` separately. Now, as far as I understand, you are supposed to add methods to the `Base.Broadcast` machinery (although it does not seem to be documented, so I am not sure if it is the proper way). I managed to get it working to some extent, but encountered a problem. Let me illustrate:

```julia
struct A
    x
end

Base.Broadcast.containertype(::A) = A
Base.Broadcast.promote_containertype(::Type{A}, ::Type{A}) = A
Base.Broadcast.promote_containertype(::Type{A}, x) = A
Base.Broadcast.promote_containertype(x, ::Type{A}) = A
Base.Broadcast.promote_containertype(::Type{A}, ::Type{Array}) = A
Base.Broadcast.promote_containertype(::Type{Array}, ::Type{A}) = A

function Base.Broadcast.broadcast_c(op, ::Type{A}, a...)
    [Symbol(".", op), a...]
end

a = A("a")
b = A("b")
c = A("c")
s = 5

println("a .+ b: $(a .+ b)")
println("a .+ s: $(a .+ s)")
println("a .+ [1 2 3]: $(a .+ [1 2 3])")

println("a .+ 1: $(a .+ 1)")
println("a .+ b .+ c: $(a .+ b .+ c)")

```

The first three expressions work fine:

```julia
a .+ b: Any[:.+, A("a"), A("b")]
a .+ s: Any[:.+, A("a"), 5]
a .+ [1 2 3]: Any[:.+, A("a"), [1 2 3]]

```

For the last two, on the other hand, there is a problem. As far as I understand, during lowering a broadcasted operator with a literal, or a sequence of several broadcasted operators is transformed right away into an anonymous function (`x -> x + 1` and `(a, b, c) -> a + b + c`), so `broadcast_c` receives this function as `op` instead of `+`:

```julia
a .+ 1: Any[Symbol(".#1"), A("a")]
a .+ b .+ c: Any[Symbol(".#3"), A("a"), A("b"), A("c")]

```

This means that the information about the actual operation is now hidden in this anonymous function, and I cannot analyze/transform the resulting expression tree, which I would like to do eventually. Is there a way to stop this from happening? For the last two cases, I would like to get something like

```julia
a .+ 1: Any[:.+, A("a"), 1]
a .+ b .+ c: Any[:.+, A("a"), A("b"), A("c")]

```

or

```julia
a .+ 1: Any[:.+, A("a"), 1]
a .+ b .+ c: Any[:.+, Any[:.+, A("a"), A("b")], A("c")]

```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [June 28, 2017, 3:44am UTC](https://discourse.julialang.org/t/preventing-broadcast-from-creating-anonymous-functions-on-lowering-for-a-custom-type/4508/2 "2017-06-28T03:44:16Z")

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[https://github.com/JuliaLang/julia/issues/22060](https://github.com/JuliaLang/julia/issues/22060)

[https://github.com/JuliaLang/julia/pull/22063](https://github.com/JuliaLang/julia/pull/22063)

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<div class="post-metadata">

**Author:** ![fjarri](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fjarri/32/630_2.png) [@fjarri](https://discourse.julialang.org/u/fjarri)\
**Post date:** [June 28, 2017, 4:38am UTC](https://discourse.julialang.org/t/preventing-broadcast-from-creating-anonymous-functions-on-lowering-for-a-custom-type/4508/3 "2017-06-28T04:38:16Z")

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Perfect, thanks!