# Access to struct fields in generated function
**URL:** https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800
**Category:** General Usage
**Created:** [October 31, 2017, 12:12pm UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800 "2017-10-31T12:12:30Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Philippe\_Maincon](https://discourse.julialang.org/u/Philippe_Maincon)
#### Post date: [October 31, 2017, 12:12pm UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800/1 "2017-10-31T12:12:30Z")
Hi,
I am trying to wrap my skull around metaprogramming. Feels like a stiff skull.
In my little example, I want a function that can take any composite type, and do some manipulation (here printing) of each field. Here we go
```julia
using StaticArrays
@generated function insight(f)
print("Generating insight.\n")
e = quote end
for fi in fieldnames(f)
blk = quote
print(string(fi),"::",string(typeof(f.$fi))," → ",show(f.$fi),"\n")
end
append!(e.args, blk.args)
end
return e
end
struct typo{nx}
σ::SMatrix{nx,nx}
b
c
end
nx = 3
i = typo{nx}(randn(nx,nx),π,3.)
insight(i)
```
I get “undefVarError: fi not defined.”
I know a “generated function” has access to type info, not values, at … generation time. I reckon fieldnames should be fair game. Are the fieldnames known at generation time? Is that the cause of the error anyway?
I also welcome comments on the way I append stuff. Is there simpler?
Philippe
---
### Author:  [@cstjean](https://discourse.julialang.org/u/cstjean)
#### Post date: [October 31, 2017, 12:30pm UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800/2 "2017-10-31T12:30:41Z")
You need to think about what computation happens when:
```julia
print(string(fi),
"::",string(typeof(f.$fi))," → ",
show(f.$fi),"\n")
```
The first `string(fi)` is quoted, so it will be evaluated when the (generated) function is called. `fi` is not a variable in your generated function, so it’s an error. You probably want `$(string(fi))` to evaluate it at generation-time.
The second `string` should work fine. But it will be performing the computation at runtime, since only `$fi` will be evaluated at generation time. You could use `$(string(fieldtype(f, fi)))` to do it at compile-time (though note that it’s not quite the same computation — your version shows the type of the value stored, but my version shows the type of the field)
The third `string` is fine.
---
### Author:  [@Philippe\_Maincon](https://discourse.julialang.org/u/Philippe_Maincon)
#### Post date: [October 31, 2017, 12:51pm UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800/3 "2017-10-31T12:51:09Z")
Thank you, that helps! One way to put it is:
```julia
@generate function foo(data)
generation time code
return quote
runtime code $(generation time code)
end
```
My code now works
```julia
using StaticArrays
@generated function insight(f)
print("Generating insight.\n")
e = quote end
for fi in fieldnames(f)
blk = quote
print($(string(fi))," :: ",$(string(fieldtype(f,fi)))," → ",string(f.$fi),"\n")
end
append!(e.args, blk.args)
end
return e
end
```
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [October 31, 2017, 3:14pm UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800/4 "2017-10-31T15:14:09Z")
Just a minor correction:
```julia
@generated function foo(data)
runtime code
return quote
runtime code $(generation time code)
end
end
```
should instead be
```julia
@generated function foo(data)
generation time code
return quote
runtime code $(generation time code)
end
end
```
---
### Author:  [@Philippe\_Maincon](https://discourse.julialang.org/u/Philippe_Maincon)
#### Post date: [November 1, 2017, 9:38am UTC](https://discourse.julialang.org/t/access-to-struct-fields-in-generated-function/6800/5 "2017-11-01T09:38:26Z")
Indeed! I corrected my post.