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Today's Topics:
1. Unmaybe (Adrian May)
2. How to improve lazyness of a foldl (and memory footprint)
(Giacomo Tesio)
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Message: 1
Date: Tue, 14 May 2013 17:21:44 +0800
From: Adrian May <[email protected]>
Subject: [Haskell-beginners] Unmaybe
To: "[email protected]" <[email protected]>
Message-ID:
<cad-ubzgs7eeeez9vuukmx7iodaksglgmknzpojkpzu_ayfo...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi there,
I have a really annoying scrap of code:
unmaybe Nothing = mempty
unmaybe (Just dia) = dia
It happened because I'm using Diagrams but building my diagram requires
looking something up in a list using findIndex, which returns Maybe Int.
How do I rid myself of this blotch?
TIA,
Adrian.
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Message: 2
Date: Tue, 14 May 2013 11:22:27 +0200
From: Giacomo Tesio <[email protected]>
Subject: [Haskell-beginners] How to improve lazyness of a foldl (and
memory footprint)
To: "[email protected]" <[email protected]>
Message-ID:
<CAHL7psF99L7YM1k=c70f1ovsch11b8wb9rbn8z4+ng7wux_...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi, I'm trying to improve a small haskell program of mine.
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
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