[BREAKING] new design of select, transform and combine

src/groupeddataframe/splitapplycombine.jl

@@ -258,9 +258,11 @@ const KWARG_PROCESSING_RULES =
     combine(fun::Union{Function, Type}, df::AbstractDataFrame, ungroup::Bool=true)
     combine(pair::Pair, df::AbstractDataFrame, ungroup::Bool=true)
 
-Apply operations to each group in a [`GroupedDataFrame`](@ref) and return
-the combined result as a `DataFrame`.
-If an `AbstractDataFrame` is passed, apply operations to the data frame as a whole.
+Apply operations to each group in a [`GroupedDataFrame`](@ref) and return the combined
+result as a `DataFrame` if `ungroup=true` or `GroupedDataFrame` if `ungroup=false`.
+
+If an `AbstractDataFrame` is passed, apply operations to the data frame as a whole
+and a `DataFrame` is always returend.
 
 $F_ARGUMENT_RULES
 
@@ -1423,9 +1425,15 @@ end
     select(gd::GroupedDataFrame, args...;
            copycols::Bool=true, keepkeys::Bool=true, ungroup::Bool=true)
 
-Apply `args` to `gd` following the rules described in [`combine`](@ref).
-The returned object has as many rows as `parent(gd)`.
-If an operation returns a single value it is always broadcasted to have this number of rows.
+Apply `args` to `gd` following the rules described in [`combine`](@ref) and return the
+result as a `DataFrame` if `ungroup=true` or `GroupedDataFrame` if `ungroup=false`.
+
+The `parent` of the returned value has as many rows as `parent(gd)`. If an operation
+in `args` returns a single value it is always broadcasted to have this number of rows.
+
+Apply operations to each group in a [`GroupedDataFrame`](@ref) and return the combined
+result as a `DataFrame` if `ungroup=true` or `GroupedDataFrame` if `ungroup=false`.
+
 
 If `copycols=false` then do not perform copying of columns that are not transformed.