# Call a macro from within a funciton

**URL:** https://discourse.julialang.org/t/call-a-macro-from-within-a-funciton/42310
**Category:** New to Julia
**Created:** [June 30, 2020, 3:41pm UTC](https://discourse.julialang.org/t/call-a-macro-from-within-a-funciton/42310 "2020-06-30T15:41:44Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![erlebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erlebach/32/12973_2.png) [@erlebach](https://discourse.julialang.org/u/erlebach)
#### Post date: [June 30, 2020, 3:41pm UTC](https://discourse.julialang.org/t/call-a-macro-from-within-a-funciton/42310/1 "2020-06-30T15:41:44Z")

</div>

I am working with `DataFrames.jl` and `DataFramesMeta.jl` (which I love).  
So consider the following MWE, not directly related to my actual work:

```julia
using DataFrames, DataFramesMeta
grps = groupby(dff, :col1)
dff = DataFrame([[1,2,3],[4,5,6]], [:col1,:col2])
dd = @based_on(grps, count=length(:col2))

```

which produces the correct result. So of course, I wish to put this procedure into a function as follows:

```julia
function find(df::DataFrame, group_col::Symbol, selector::Symbol)
    grps = groupby(df, group_col)
    w_count_df = @based_on(grps, count=length(group_col))
end

dff = DataFrame([[1,2,3],[4,5,6]], [:col1,:col2])
dd = find(dff, :col1, :col2)

```

which produces an error because the macro expansion occurs before the call to the function:

```julia
julia> dd = find(dff, :col1, :col2)
ERROR: MethodError: no method matching length(::Symbol)
Closest candidates are:
  length(::Core.SimpleVector) at essentials.jl:596
  length(::Base.MethodList) at reflection.jl:852
  length(::Core.MethodTable) at reflection.jl:938
  ...
Stacktrace:
 [1] (::var"#288#289"{Symbol})(::SubDataFrame{DataFrame,DataFrames.Index,Array{Int64,1}}) at /Users/erlebach/.julia/packages/DataFramesMeta/c67UK/src/DataFramesMeta.jl:80
 [2] _combine(::var"#288#289"{Symbol}, ::GroupedDataFrame{DataFrame}, ::Nothing, ::Bool, ::Bool) at /Users/erlebach/.julia/packages/DataFrames/kwVTY/src/groupeddataframe/splitapplycombine.jl:1215
 [3] combine_helper(::Function, ::GroupedDataFrame{DataFrame}, ::Nothing; keepkeys::Bool, ungroup::Bool, copycols::Bool, keeprows::Bool) at /Users/erlebach/.julia/packages/DataFrames/kwVTY/src/groupeddataframe/splitapplycombine.jl:583
 [4] #combine#375 at /Users/erlebach/.julia/packages/DataFrames/kwVTY/src/groupeddataframe/splitapplycombine.jl:434 [inlined]
 [5] combine at /Users/erlebach/.julia/packages/DataFrames/kwVTY/src/groupeddataframe/splitapplycombine.jl:434 [inlined]
 [6] find(::DataFrame, ::Symbol, ::Symbol) at /Users/erlebach/covid_modeling_julia/julia_code/sir-julia/SIRsims/gordon_julia_code/household_workplaces_leon/drawNetwork.jl:22
 [7] top-level scope at none:0

```

So how would I make this work, WITHOUT modifying the `@based_on` macro? I can’t go make changes to existing macros every time I want to call a macro. I have read as much as I could understand. Thank you.

---

<div class="post-metadata">

### Author: ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [June 30, 2020, 3:54pm UTC](https://discourse.julialang.org/t/call-a-macro-from-within-a-funciton/42310/2 "2020-06-30T15:54:30Z")

</div>

Not an answer to your question, but especially now with `combine`, `select` and `transform` in core DataFrames the answer to problems with DataFramesMeta can often be just don’t use it…

Your actual problem might of course be trickier, but from your MWE it seems that

```julia
 combine(groupby(dff, :col1), :col2 => length => :count)

```

does the trick