# Spurious parentheses in generated expression

**URL:** <https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572>\
**Category:** General Usage\
**Tags:** syntax\
**Created:** [June 10, 2022, 4:22pm UTC](https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572 "2022-06-10T16:22:26Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Mark\_Nahabedian](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mark_nahabedian/32/16562_2.png) [@Mark\_Nahabedian](https://discourse.julialang.org/u/Mark_Nahabedian)\
**Post date:** [June 10, 2022, 4:22pm UTC](https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572/1 "2022-06-10T16:22:26Z")

</div>

I have a regression test that’s failing. It’s testing a helper function to a macro that adds a `source` slot to a struct definition and modifies the constructor methods appropriately.

That helper function returns this expression

```julia
Base.@ __doc__ (struct Sequence <: BNFNode
            source::(Union){Nothing, LineNumberNode}
            elements::Tuple{Vararg{<:BNFNode}}
            function Sequence(elements...)
                new(nothing, elements)
            end
            function Sequence(source::Union{Nothing, LineNumberNode}, elements...)
                new(source, elements)
            end
        end) == 

```

The text expects it to return

```julia
Base.@ __doc__ (struct Sequence <: BNFNode
            source::Union{Nothing, LineNumberNode}
            elements::Tuple{Vararg{<:BNFNode}}
            function Sequence(elements...)
                new(nothing, elements)
            end
            function Sequence(source::Union{Nothing, LineNumberNode}, elements...)
                new(source, elements)
            end
        end)

```

The only difference I see is the Parentheses around `Union` in the `source` slot. I’d think those parentheses are meanaingless though and I don’t know why they’re there.

Note that the results of parsing the slot definition both with and without those parentheses are identical:

```julia
Meta.@dump(source::(Union){Nothing, LineNumberNode})
Expr
  head: Symbol ::
  args: Array{Any}((2,))
    1: Symbol source
    2: Expr
      head: Symbol curly
      args: Array{Any}((3,))
        1: Symbol Union
        2: Symbol Nothing
        3: Symbol LineNumberNode

Meta.@dump(source::Union{Nothing, LineNumberNode})
Expr
  head: Symbol ::
  args: Array{Any}((2,))
    1: Symbol source
    2: Expr
      head: Symbol curly
      args: Array{Any}((3,))
        1: Symbol Union
        2: Symbol Nothing
        3: Symbol LineNumberNode

```

One of my frustrations with Julia is the number of different syntaxes and the non-obvious (to me) correspondences between them.

What can I do to my test to make it not sensitive to this peculiar and pointless difference?

The test is at [AnotherParser.jl/test\_note\_BNFNode\_location.jl at a5b3496b0f7f45f3a65479b3dc4f99c2e964c790 · MarkNahabedian/AnotherParser.jl · GitHub](https://github.com/MarkNahabedian/AnotherParser.jl/blob/a5b3496b0f7f45f3a65479b3dc4f99c2e964c790/test/test_note_BNFNode_location.jl#L5)

---

<div class="post-metadata">

**Author:** ![cjdoris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cjdoris/32/213133_2.png) [@cjdoris](https://discourse.julialang.org/u/cjdoris)\
**Post date:** [June 10, 2022, 4:32pm UTC](https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572/2 "2022-06-10T16:32:31Z")

</div>

I believe the difference is this:

```julia-repl
julia> a = :(Union{Foo,Bar})
:(Union{Foo, Bar})

julia> b = :($Union{Foo,Bar})
:((Union){Foo, Bar})

julia> a.args[1]
:Union

julia> b.args[1]
Union

```

i.e. one of your expressions interpolates `$Union` into the expression, whereas the other one just writes (the symbol) `Union` directly.

This means they are indeed different expressions: you could evaluate `a` in a module where `Union = Dict` is defined and you’d get a `Dict{Foo,Bar}`, whereas evaluating `b` will always give `Union{Foo,Bar}`.

The parentheses are intentional, to distinguish code from interpolated values (which look like code).

---

<div class="post-metadata">

**Author:** ![Mark\_Nahabedian](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mark_nahabedian/32/16562_2.png) [@Mark\_Nahabedian](https://discourse.julialang.org/u/Mark_Nahabedian)\
**Post date:** [June 10, 2022, 4:37pm UTC](https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572/3 "2022-06-10T16:37:36Z")

</div>

Wow. Thanks for your quick response.

I don’t think that’sit though as the slot definition expression is constant:

```julia
        push!(constructors2,
              :(function $(bindings2[:fname])(source::Union{Nothing, LineNumberNode},
                                              $(bindings2[:fargs]...))
                    $(map(bindings2[:fbody]) do e
                          postwalk(e) do e
                              if iscall(e, :new)
                                  Expr(:call, :new, :source, e.args[2:end]...)
                              else
                                  e
                              end
                          end
                      end...)
                end))

```

---

<div class="post-metadata">

**Author:** ![Mark\_Nahabedian](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mark_nahabedian/32/16562_2.png) [@Mark\_Nahabedian](https://discourse.julialang.org/u/Mark_Nahabedian)\
**Post date:** [June 10, 2022, 4:51pm UTC](https://discourse.julialang.org/t/spurious-parentheses-in-generated-expression/82572/4 "2022-06-10T16:51:58Z")

</div>

Dooo. That wasn’t the right bit of code.

The problem was here

```julia
Expr(:curly, :Union, Nothing, LineNumberNode)),

```

where `Union` wasn’t quoted.

Thanks.