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copied!<p><code>map</code> - change each element</p> <p><code>fold</code> - combine all the elements</p> <p><code>scan</code> - combine all the elements keeping a running "total" - this is what you need</p> <p>It's going to be easier to keep everything as an Integer until the very end:</p> <pre><code>type PosixOffset = Integer </code></pre> <p>A string in your <code>listOfTimes</code> could be a unix time, an increment or an erroneous value. We could represent that by <code>Maybe (Either PosixOffset Integer)</code> but that could get annoying. Let's roll our own:</p> <pre><code>data Time = Unix PosixOffset | Inc Integer | Error String deriving Show </code></pre> <p>This allows me to be flexible about what we do later with an error: crash the program with an <code>error</code>, show the <code>Error</code> message to the user but somehow allow them to resume, or ignore the bad value.</p> <p>Let's make safe version to replace <code>read :: String -> Integer</code>, which returns <code>Nothing</code> instead of crashing. We'll need to <code>import Data.Char (isDigit)</code></p> <pre><code>readInteger :: String -> Maybe Integer readInteger "" = Nothing readInteger xs | all isDigit xs = Just (read xs) | otherwise = Nothing </code></pre> <p>Now we can use that to <code>readTime</code> with some helpful <code>Error</code> messages.</p> <pre><code>readTime :: String -> Time readTime ('u':xs) = case readInteger xs of Just i -> Unix i Nothing -> Error $ "readTime: there should be an integer after the u, but I got: " ++ 'u':xs readTime [] = Error "readTime: empty time" readTime xs = case readInteger xs of Just i -> Inc i Nothing -> Error $ "readTime: " ++ xs ++ " is neither a unix time nor an increment." </code></pre> <p>The plan is to convert our list of Strings to a list of pairs <code>(PosixOffset,Integer)</code>, with the last known <code>PosixOffset</code> from a unix time, and the current increment. We'll then need to be able to convert these pairs to a <code>UTCTime</code> </p> <pre><code>toUTC :: (PosixOffset,Integer) -> UTCTime toUTC (p,i) = psUTC (p+i) </code></pre> <p>Now we need to know how to combine the running total of the <code>Time</code>s with the next <code>Time</code>. We'll keep hold of the last unix time for reference.</p> <pre><code>addTime :: (PosixOffset,Integer) -> Time -> (PosixOffset,Integer) addTime (oldunix,oldinc) time = case time of Unix new -> (new,0) -- If there's a new unix time, replace and reset the inc to 0. Inc inc -> (oldunix,inc) -- If there's a new increment, replace the old one. Error msg -> error msg -- If there's an error, crash showing it. </code></pre> <p>or you could use </p> <pre><code>addTimeTolerant :: (PosixOffset,Integer) -> Time -> (PosixOffset,Integer) addTimeTolerant (oldunix,oldinc) time = case time of Unix new -> (new,0) -- If there's a new unix time, replace and reset the inc to 0. Inc inc -> (oldunix,inc) -- If there's a new increment, replace the old one. Error msg -> (oldunix,oldinc) -- If there's an error, ignore it and keep the time the same. </code></pre> <p>Now we can stick it together: turn the <code>String</code>s into <code>Time</code>s, then combine them into <code>(PosixOffset,Integer)</code> pairs by <code>scan</code>ning with <code>addTime</code>, then turn all the resulting pairs into <code>UTCTime</code>s.</p> <pre><code>runningTotal :: [String] -> [UTCTime] runningTotal [] = [] runningTotal xss = let (t:ts) = map readTime xss in -- turn Strings to Times case t of Error msg -> error msg Inc _ -> error "runningTotal: list must start with a unix time" Unix po -> map toUTC $ scanl addTime (po,0) ts -- scan the list adding times, -- starting with an initial unix time -- then convert them all to UTC </code></pre> <p>or if you like the keep calm and carry on approach of <code>addTimeTolerant</code>, you could use</p> <pre><code>isn't_UnixTime :: Time -> Bool isn't_UnixTime (Unix _) = False isn't_UnixTime _ = True runningTotalTolerant :: [String] -> [UTCTime] runningTotalTolerant xss = let ts = dropWhile isn't_UnixTime (map readTime xss) in -- cheerily find the first unix time if null ts then [] else -- if there wasn't one, there are no UTCTimes let (Unix po) = head ts in -- grab the first time map toUTC $ scanl addTimeTolerant (po,0) (tail ts) -- scan the list adding times, -- starting with an initial unix time -- then convert them all to UTC </code></pre>

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