Tim Docker wrote:

...

data ColDesc rowv = forall a. ColDesc (rowv -> a) ([a] -> a) (a -> String)

calculate :: [rowv] -> ColDesc rowv -> ([String],String) calculate rs (ColDesc valf sumf fmtf) = let vals = map valf rs in (map fmtf vals, (fmtf.sumf) vals)

This code only does the (rowv -> a) evaluation once

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

data Results = Results { formatted:: [String], aggregated:: String }

type ColDesc rowv = [rowv] -> Results

calculate :: [rowv] -> ColDesc rowv -> ([String],String) calculate rs transformer = (formatted res, aggregated res) where res = transformer rs

rows = ["I","wish","to","define","a","report"]

mapp g f = g . (map f)

cols = [ \rvs -> Results rvs "", \rvs -> let vals = map length rvs in Results (map show vals) (show$sum vals), \rvs -> let vals = map (\s -> length s * length s) rvs in Results (map show vals) (show$maximum vals) ]

values = [ calculate rows col | col <- cols ]

If we just need to format one value, we can write

format_value v transformer = formatted $ transformer [v]

Due to laziness, the aggregate function will not be computed, unless specifically requested.