# Can I use \`@eval\` to dynamically define methods for dispatch on \`Val\`s?
**URL:** https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468
**Category:** General Usage
**Tags:** question, metaprogramming
**Created:** [September 20, 2021, 2:45pm UTC](https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468 "2021-09-20T14:45:31Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@kapple](https://discourse.julialang.org/u/kapple)
#### Post date: [September 20, 2021, 2:45pm UTC](https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468/1 "2021-09-20T14:45:31Z")
Hi all,
I’m trying to dynamically define methods for a function to dispatch on specific names (symbols/strings) which seems to work outside a loop, but not inside a loop.
Can someone explain how I may be mis-applying the metaprogramming functionality of Julia here?
```julia-repl
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.6.0 (2021-03-24)
_/ |\ __'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |
julia> for name in [:aye, :bee, :cee]
@eval Main begin
function fcn(::Val{name})
println(name)
end
end
end
ERROR: UndefVarError: name not defined
Stacktrace:
[1] top-level scope
@ REPL[1]:3
[2] eval(m::Module, e::Any)
@ Core .\boot.jl:360
[3] top-level scope
@ REPL[1]:2
julia> new_name = :dee
:dee
julia> @eval Main begin
function gcn(::Val{new_name})
println(new_name)
end
end
gcn (generic function with 1 method)
julia> gcn(Val(:dee))
dee
```
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [September 20, 2021, 2:48pm UTC](https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468/2 "2021-09-20T14:48:09Z")
You need to escape `name`, so
```julia
for name in [:aye, :bee, :cee]
@eval Main begin
function fcn(::Val{$name})
println($name)
end
end
end
```
Otherwise it is trying to look up `name` when you run the method instead of when you define it.
---
### Author:  [@kapple](https://discourse.julialang.org/u/kapple)
#### Post date: [September 22, 2021, 12:37am UTC](https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468/3 "2021-09-22T00:37:52Z")
My apologies, I forgot to mention that I did try this as well, and it didn’t work.
```julia
julia> for name in [:aye, :bee, :cee]
@eval Main begin
function fcn(::Val{$name})
println($name)
end
end
end
ERROR: UndefVarError: aye not defined
Stacktrace:
[1] top-level scope
@ REPL[1]:3
[2] eval
@ .\boot.jl:360 [inlined]
[3] top-level scope
@ .\REPL[1]:2
```
Thank you though! I figured it out and yeah what you suggested is one step closer, this is what works:
```julia
julia> for name in [:(:aye), :(:bee), :(:cee)]
@eval Main begin
function fcn(::Val{$name})
println($name)
end
end
end
julia> fcn(Val(:aye))
aye
```
It’s just strange that is complains that `name` is undefined in my OP example.
---
### Author:  [@Per](https://discourse.julialang.org/u/Per)
#### Post date: [September 22, 2021, 7:21am UTC](https://discourse.julialang.org/t/can-i-use-eval-to-dynamically-define-methods-for-dispatch-on-val-s/68468/4 "2021-09-22T07:21:57Z")
Another option is:
```julia
for name in [:aye, :bee, :cee]
@eval Main begin
function fcn(::Val{$(QuoteNode(name))})
println($(QuoteNode(name)))
end
end
end
```