Combine function not naming column as cols => function => target_cols implies

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The Pair(s) are resolved before being passed to the combine function. Let’s look below at what the combine function is receiving as its second argument.

Your original version is a single Pair from the column name to a function output. (The “var” thing is denoting an anonymous function.) The output of the anonymous function is a Pair, which you can see in your provided PetalLength_function column screenshot.

julia> :PetalLength => x -> mean(x) => :myMean
:PetalLength => var"#1#2"()

Julia reads the above the same way as if you would have put the parentheses around the final Pair. Written this way, you have only provided combine with a source column and an operation function.

julia> :PetalLength => x -> (mean(x) => :myMean)
:PetalLength => var"#13#14"()

julia> typeof(:PetalLength => x -> (mean(x) => :myMean))
Pair{Symbol, var"#17#18"}

You need to separate the operation function from the new column name with parentheses so that combine sees nested Pairs as its second argument.

julia> :PetalLength => (x -> mean(x)) => :myMean
:PetalLength => (var"#3#4"() => :myMean)

julia> typeof(:PetalLength => (x -> mean(x)) => :myMean)
Pair{Symbol, Pair{var"#5#6", Symbol}}

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