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    <p>Ok, so between <code>BEGIN:VEVENT</code> and <code>END:VEVENT</code>, you have many key value pairs. So write a rule <code>keyValuePair</code> that returns <code>(key, value)</code>. Now inside the rule for <code>VEVENT</code> you do <code>many KeyValuePair</code> to get a list of pairs. Once you've done that you use a fold to populate a VEVENT record with the given values. In the function you give to fold, you use pattern matching to find out in which field to store the value. As the starting value for the accumulator you use a VEvent record where the optional fields are set to <code>Nothing</code>. Example:</p> <pre><code>pairs &lt;- many keyValuePairs vevent = foldr f (VEvent {sequence = Nothing}) pairs where f ("SUMMARY", v) ve = ve {summary = v} f ("DSTART", v) ve = ve {dstart = read v} </code></pre> <p>...and so on. Do the same for the other components.</p> <p>Edit: Here's some runnable example code for the fold:</p> <pre><code>data VEvent = VEvent { summary :: String, dstart :: String, sequenceSt :: Maybe String } deriving Show vevent pairs = foldr f (VEvent {sequenceSt = Nothing}) pairs where f ("SUMMARY", v) ve = ve {summary = v} f ("DSTART", v) ve = ve {dstart = v} f ("SEQUENCEST", v) ve = ve {sequenceSt = Just v} main = do print $ vevent [("SUMMARY", "lala"), ("DSTART", "lulu")] print $ vevent [("SUMMARY", "lala"), ("DSTART", "lulu"), ("SEQUENCEST", "lili")] </code></pre> <p>Output:</p> <pre><code>VEvent {summary = "lala", dstart = "lulu", sequenceSt = Nothing} VEvent {summary = "lala", dstart = "lulu", sequenceSt = Just "lili"} </code></pre> <p>Note that this will produce a warning when compiled. To avoid the warning, initialize all non-optional fields to <code>undefined</code> explicitly.</p>
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    1. COThis is similar to what I thought of at first but how do you decide what 'zero' is for example? You gave the 'zero' as 'VEvent {sequence = Nothing}' but what if the structure is more complicated like one of the ones that I made, what if it has a bunch of integer fields that have to be set and you don't want to make every record field a Maybe. Can you make records piece by piece with multiple required fields that you do not give at the start? (Let me know if I need to describe this new question better and I'll try to provide a code example)
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    2. CO@Robet: As I said, initialize the optional ones to `Nothing`. The other ones you don't need to initialize at all. If there were no optional ones `Vevent {}` would be a perfectly fine starting value (or you could explicitly set every non-optional value to undefined to avoid getting a warning, but I suppose that would get tiresome with a lot of fields).
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    3. COThis fails for me though? http://pastie.org/1157529
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