oleg wrote:

Sorry I didn't know of that requirement. The desired sharing can easily be introduced:

That's always a risk with posting a toy example - which bits are important can be hard to tell. You are right of course about being able to introduce the sharing. FWIW, after the suggestions that I've had (thanks!), my "existential-free" example is as below. Tim type ColumnFn rowv = [rowv] -> ([String],String) column :: (rowv->a) -> ([a]->a) -> (a->String) -> ColumnFn column valf sumf fmtf rows = (map fmtf vals, (fmtf.sumf) vals) where vals = map valf rows calculate :: [ColumnFn rowv] -> [rowv] -> ([String],String) calculate cfs rows = [ cf rows | cf <- cfs ] ---- rows = ["I","wish","to","define","a","report"] cols = [ column id (\_ -> "") id, column length sum show, column (\s -> length s * length s) maximum show ] :r values = [ calculate rows col | col <- cols ]