# Is there a \`with\` operator or usage pattern?

**URL:** <https://discourse.julialang.org/t/is-there-a-with-operator-or-usage-pattern/52693>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [January 1, 2021, 1:05pm UTC](https://discourse.julialang.org/t/is-there-a-with-operator-or-usage-pattern/52693 "2021-01-01T13:05:06Z")\
**Posts on this page:** 1\
**Showing post:** 13

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [January 1, 2021, 8:36pm UTC](https://discourse.julialang.org/t/is-there-a-with-operator-or-usage-pattern/52693/13 "2021-01-01T20:36:18Z")

</div>

Note that this doesn’t work at all within a function:

```julia
julia> function f(subtotal)
         @with subtotal begin
           price + shipping
         end
       end
f (generic function with 1 method)

julia> f(subtotal)
ERROR: MethodError: no method matching (::Base.Meta.var"#14#16")(; price=12.99, shipping=3.99)
Closest candidates are:
  #14(; price, shipping) at REPL[1]:16
Stacktrace:
 [1] macro expansion at ./REPL[1]:9 [inlined]
 [2] f(::NamedTuple{(:price, :shipping),Tuple{Float64,Float64}}) at ./REPL[8]:2
 [3] top-level scope at REPL[9]:1

```

and for the same reason it cannot be benchmarked with `@btime` from BenchmarkTools.

For a much simpler solution which works in local and global scope and imposes no performance penalty at all, see: [With struct do - #12 by rdeits](https://discourse.julialang.org/t/with-struct-do/41341/12)

For example:

```julia
julia> using MacroTools

julia> using MacroTools: postwalk

julia> macro with(container, expr)
         esc(postwalk(expr) do ex
           if @capture(ex, &field_)
             :(($container).$field)
           else
             ex
           end
         end)
       end
@with (macro with 1 method)

julia> subtotal = (price=12.99, shipping=3.99)
(price = 12.99, shipping = 3.99)

julia> @with subtotal begin
         &price + &shipping
       end
16.98

julia> function f(subtotal)
         @with subtotal begin
           &price + &shipping
         end
       end
f (generic function with 1 method)

julia> f(subtotal)
16.98

julia> using BenchmarkTools

julia> @btime f($subtotal)
  0.029 ns (0 allocations: 0 bytes)
16.98

```

Note that the suspiciously-fast benchmark time of \< 1ns means that the compiler has actually eliminated all of the code in the benchmark and just replaced it with the answer. That’s actually great news for us–it means that the revised `@with` macro doesn’t make the code any harder for the compiler to analyze and optimize.

---

_[View the full topic](https://discourse.julialang.org/t/is-there-a-with-operator-or-usage-pattern/52693)._