I think I need a monad but I'm not sure which, and maybe something simpler would do.

I want to model a fish tank as a function of time onto how many fish there are in it at that time. If I already have such a function, I want to operate on it with something like "add 2 fish on day 20" or "take 3 away on day 15" to get a new function of the same form, but the latter should not remove more fish than there are in the tank at that time, and it should tell me how many I get. I don't promise to apply these operators in chronological order.

This seems like the kind of thing that would be in the prelude somewhere. But where?

TIA,

Adrian.

and ran HLint on it
$ ~/.cabal/bin/hlint hlint_test.hs
No suggestions

  fn :: [a] -> a

  fn = (head .) . takeWhile $ (\_ -> True)

From this, I want a function with the signature fn' :: [(x,a)] -> (x,a) such that:

    snd $ fn' (zip [x] [a]) = fn [a]

I can see ways of doing this by altering fn, or by repeating some of fn in the definition of fn', or (because in this case I know that if fn xs = x, fn is returning the first x in xs and not any others), by doing something nasty like:

  fn' xs = xs !! fromMaybe 0 (findIndex (\(_,a) -> a == fn (snd $ unzip xs)) xs )

But it seems to me like there should be prettier solutions to this (that do not involve changing the definition of fn). After all, this doesn't seem like a rare pattern.

Anyone know if in fact there's a better way to go about it?

  fn :: [a] -> a

  fn = (head .) . takeWhile $ (\_ -> True)

From this, I want a function with the signature fn' :: [(x,a)] -> (x,a) such that:

    snd $ fn' (zip [x] [a]) = fn [a]

I can see ways of doing this by altering fn, or by repeating some of fn in the definition of fn', or (because in this case I know that if fn xs = x, fn is returning the first x in xs and not any others), by doing something nasty like:

  fn' xs = xs !! fromMaybe 0 (findIndex (\(_,a) -> a == fn (snd $ unzip xs)) xs )

But it seems to me like there should be prettier solutions to this (that do not involve changing the definition of fn). After all, this doesn't seem like a rare pattern.

Anyone know if in fact there's a better way to go about it?

I'm trying to draw a picture with diagrams (this isn't the gantt chart I was talking about before.)

A more extended description with full source code is here: http://codereview.stackexchange.com/questions/26107/how-to-improve-readability-and-memory-footprint-of-this-haskell-script

The script transforms CSV files into other CSV files but looks like it's reading the whole input files before writing output files.

I guess that the script can be improved in many ways, in readability and efficiency, thus any suggestion is wellcome as an occasion to learn.

But what I can't understand is why this design doesn't work:

transformFile :: FilePath -> ([String] -> a) -> (a -> IO r) -> IO r transformFile file operation continuation = withFile file ReadMode (\h -> hGetContents h >>= (continuation.operation.lines))

This function recieves a path, a function to left fold lines to a new list of objects and a function to persist the fold output to files.

Here the relevant parts:

importTrades :: FilePath -> FilePath -> IO () importTrades outDir csvFile = transformFile csvFile (foldTradingSample.getTickWriteTrades) (saveTradingSamples outDir) where getTickWriteTrades = filter (isBetween (9, 0) (18, 0)).(catMaybes.(map fromCSVLine)) foldTradingSample = foldl toTradingSample []

This is the folding function:

toTradingSample :: [TradingSample] -> Tw.Trade -> [TradingSample] toTradingSample (current:others) twTrade | newEqt == equity current && newDay == day current = (current { trades = newTrades }):others | otherwise = current : toTradingSample others twTrade where newEqt = Tw.tSimbol twTrade newDay = Tw.tDate twTrade newTrade = fromTickWrite twTrade newTrades = trades current ++ [newTrade] toTradingSample [] twTrade = [TradingSample { equity = Tw.tSimbol twTrade , day = Tw.tDate twTrade , trades = [fromTickWrite twTrade] }]

And this is the function that safe the fold results to files

saveTradingSamples :: String -> [TradingSample] -> IO () saveTradingSamples folder samples = mapM_ (saveTradingSample folder) samples saveTradingSample :: String -> TradingSample -> IO () saveTradingSample folder sample = writeFile fileName contents where fileName = folder ++ "\\" ++ (equity sample) ++ "_" ++ (formatTime defaultTimeLocale "%F" $ day sample) ++ ".CSV" contents = tradingSampleToCSV sample

What's wrong here?

My insight is that the problem is in the signature of transform files, that requires to completely compute the list of TradingSample before calling saveTradingSamples.

Is this the problem? How can I fix this?

Giacomo

I have a really annoying scrap of code:

TIA,

Adrian.