Expr eval within Julia module gives UndefVarError-CodePudding

I'm able to run the following code just fine and it provides the expected result:

julia> using DataFrames, DataFramesMeta

julia> expr = "2*:a :b"
"2*:a :b"

julia> df = DataFrame(a=[1,2],b=[3,4])
2×2 DataFrame
 Row │ a      b     
     │ Int64  Int64 
─────┼──────────────
   1 │     1      3
   2 │     2      4

julia> eval(Meta.parse("@transform(df, " * join(collect(":res" => expr), " = ") * ")"))
2×3 DataFrame
 Row │ a      b      res   
     │ Int64  Int64  Int64 
─────┼─────────────────────
   1 │     1      3      5
   2 │     2      4      8

However, this fails when done inside a module.

julia> module foo
           using DataFrames, DataFramesMeta
           function bar()
               expr = "2*:a :b"
               df = DataFrame(a=[1,2],b=[3,4])
               eval(Meta.parse("@transform(df, " * join(collect(":res" => expr), " = ") * ")"))
           end
       end
Main.foo

julia> foo.bar()
ERROR: UndefVarError: df not defined
Stacktrace:
 [1] top-level scope
   @ ~/.julia/packages/DataFramesMeta/BkRtJ/src/macros.jl:1363
 [2] eval
   @ ./boot.jl:368 [inlined]
 [3] eval
   @ ./REPL[9]:1 [inlined]
 [4] bar()
   @ Main.foo ./REPL[9]:6
 [5] top-level scope
   @ REPL[10]:1

I imagine this is possible scope issue, and tried to be explicit by using foo.df instead of df in the function call, but without success.

Would anyone know how what is preventing df from being recognized as a defined variable here?

CodePudding user response：

The reason is that eval is evaluated in global scope, as you can read in its docstring:

Evaluate an expression in the global scope of the containing module.

In general using Meta.parse and eval is highly not recommended. Can you please explain the problem you are trying to solve as there is probably another way to handle it.

CodePudding user response：

Like Bogumil said eval totally does not look like a recommended way to write your code.

However if your goal is learning macros this is the correct code:

module Foo2
    using DataFrames, DataFramesMeta
    function bar()
        expr = "2*:a :b"
        df = DataFrame(a=[1,2],b=[3,4])
        code = quote
            @transform($df, :res = $(Meta.parse(expr)))
        end
        eval(code)
    end
end

Now you can run:

julia> Foo2.bar()
2×3 DataFrame
 Row │ a      b      res
     │ Int64  Int64  Int64
─────┼─────────────────────
   1 │     1      3      5
   2 │     2      4      8