# Override foo{x}
**URL:** https://discourse.julialang.org/t/override-foo-x/61969
**Category:** General Usage
**Created:** [May 28, 2021, 3:47am UTC](https://discourse.julialang.org/t/override-foo-x/61969 "2021-05-28T03:47:35Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@jzr](https://discourse.julialang.org/u/jzr)
#### Post date: [May 28, 2021, 3:47am UTC](https://discourse.julialang.org/t/override-foo-x/61969/1 "2021-05-28T03:47:35Z")
Is it possible to customize what happens when I do `foo{x}` for a particular type `Foo` or type `X`?
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [May 28, 2021, 5:11am UTC](https://discourse.julialang.org/t/override-foo-x/61969/2 "2021-05-28T05:11:08Z")
Nope. You could write a macro though, e.g. `@bar foo{x}` can mean whatever you want it to mean.
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [May 28, 2021, 5:20am UTC](https://discourse.julialang.org/t/override-foo-x/61969/3 "2021-05-28T05:20:16Z")
Here’s what the syntax `foo{x}` lowers to:
```julia
julia> Meta.@lower foo{x}
:($(Expr(:thunk, CodeInfo(
@ none within `top-level scope'
1 ─ %1 = Core.apply_type(foo, x)
└── return %1
))))
```
and looking at `Core.apply_type`,
```julia
julia> Core.apply_type
apply_type (built-in function)
```
we see that it is a built-in function and hence cannot have methods added to it
---
### Author:  [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [May 28, 2021, 5:36am UTC](https://discourse.julialang.org/t/override-foo-x/61969/4 "2021-05-28T05:36:23Z")
Expanding on this,
```julia
julia> e = :(foo{x})
:(foo{x})
julia> e.head
:curly
julia> e.args
2-element Vector{Any}:
:foo
:x
```
so
```julia
macro bar(e)
v = e.args[1] # vector
i = e.args[2] # index
if e.head === :curly
i = :(length(v) - $i + 1)
end
return esc(Expr(:ref, v, i))
end
```
gives you a (inefficient and difficult to understand) specialized syntax:
```julia
julia> foo = 1:10
1:10
julia> x = 2
2
julia> @bar foo[x] # get the second element
2
julia> @bar foo{x} # get the second to last element
9
```
---
### Author:  [@Elrod](https://discourse.julialang.org/u/Elrod)
#### Post date: [May 28, 2021, 5:37am UTC](https://discourse.julialang.org/t/override-foo-x/61969/5 "2021-05-28T05:37:40Z")
```julia
julia> struct Foo{T} x::T end
julia> Foo{Int}(x) = Foo{Float64}(Float64(x))
julia> Foo{Int}(3)
Foo{Float64}(3.0)
```