# Understanding macro lexical bindings
**URL:** https://discourse.julialang.org/t/understanding-macro-lexical-bindings/125105
**Category:** General Usage
**Tags:** macros, metaprogramming
**Created:** [January 23, 2025, 9:07am UTC](https://discourse.julialang.org/t/understanding-macro-lexical-bindings/125105 "2025-01-23T09:07:13Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@nietzsche](https://discourse.julialang.org/u/nietzsche)
#### Post date: [January 23, 2025, 9:07am UTC](https://discourse.julialang.org/t/understanding-macro-lexical-bindings/125105/1 "2025-01-23T09:07:13Z")
Hello,
the macroexpand in the code below lead to an Error.
`UndefVarError: i not defined`
Ther error is clear, but how can I solve it? Or how can I put the “i” from the for loop in the right environment?
```julia
macro dlamdba(body...)
quote
#println(length($(body)))
(msg) ->
begin
for i = 1:2
println((i))
if msg == $(body[i].args[1])
println(msg)
$(esc(body[i].args[2]))()
end
end
end
end
end
@macroexpand @dlamdba [:inc,() -> count = count +1] [:dec,() -> count = count -1]
```
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [January 23, 2025, 9:38am UTC](https://discourse.julialang.org/t/understanding-macro-lexical-bindings/125105/2 "2025-01-23T09:38:05Z")
`i` only exists in the quote as a symbol, it’s not a variable until after the macro returns. Executing `body[i].args[1]` before interpolation into the quote doesn’t work because there’s no `i` variable.
I think the fundamental issue is you’re trying to write a for loop that changes subexpressions per iteration. That’s not something Julia code can do, so macros can’t transform code into that. You only have so many arguments for `@dlambda`, so you could just execute them all in sequence, like a fully unrolled loop. You can’t really write that in a quote, but you can mutate the `Expr` that the quote instantiates. Write out the unrolled loop and examine the `Expr` structure with `dump` to get an idea of how to mutate towards that.
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [January 23, 2025, 12:59pm UTC](https://discourse.julialang.org/t/understanding-macro-lexical-bindings/125105/3 "2025-01-23T12:59:55Z")
While @Benny’s answer is of course true, I think a potentially more helpful answer is that you need to do the loop over `i` inside the macro, rather than leaving it as quoted code.
i.e. something like
```julia
macro dlamdba(body...)
f_body = Expr(:block)
for i ∈ 1:2
ex = quote
println($i)
if msg == $(esc(body[i].args[1]))
println(msg)
$(esc(body[i].args[2]))()
end
end
push!(f_body.args, ex)
end
:(msg -> $f_body)
end
```