Are you trying to use the whole Dataset object as an argument to dfn/mean? I don't think there's that kind of polymorphism built in - you'd have to call tablecloth.api/column or tablecloth.api/map-columns to operate on the column within the dataset.

There is no such polymorphism. You need to pass a column object, eg. (dfn/mean (ds :column-name))

So maybe my initial sin is using tc/select-colums to select only one column? That creates a Dataset, that's why I was trying to figure out if a single column becomes a "series" or something like that because it seems like that's what dfn/mean expects to operate on

select-columns creates a dataset with a subset of columns. To select a column use a dataset as a map or call tc/column . More about it here: https://scicloj.github.io/tablecloth/index.html#Columns_and_rows

dfn/mean (and any other dfn functions) operates on a dtype-next buffer, Clojure sequence or a Column object but not on a dataset itself.

Thank you, by the way. tc/column solved everything