flatMap (with a capital M)
-Ross
On Jan 18, 2010, at 5:21 PM, pabraham wrote:
> Hi Naftoli,
>
> The code I have is:
>
> def allexpenses(expenseTemplate: NodeSeq): NodeSeq = Expense.findAll(By
> (Expense.month,m.toLong)).flatmap ({ month =>
> bind("month", expenseTemplate,
> "name" -> Text( month.item ),
> "amount" -> Text( month.amount )
> )
> })
> bind("thismonth", xhtml, "expenseitem" -> allexpenses _)
>
> which gives the error: value flatmap is not a member of List
> [demo.expenses.model.Expense]
>
> From what you've said, I'm not sure how to remove the flatmap without
> generating another error. I realise this is a bit cheeky, but please
> humour me and rewrite my code above to remove the flatmap and errors.
>
> Also given an ID, how can I create an instance of a Month class whose
> ID matches that ID?
>
> Regards,
>
> Paul.
>
>
> On 18 Jan, 21:57, Naftoli Gugenheim <[email protected]> wrote:
>> val l = List(1,2,3)
>> l.map(x => x * 2) == List(2,4,6)
>> So map take as its argument a function that transforms every member of the
>> list.
>> flatmap is similar but instead of the new List *consisting of* the
>> transformed elements, it consists of the *concatenation* of the results of
>> the function (which must be concatenatable). So
>> l.flatmap(x => List(x, x * 2)) == List(1,2) ++ List(2,4) ++ List(3,6) ==
>> List(1,2,2,4,3,6)
>> Since bind returns a NodeSeq, if you would return the result of bind from a
>> function you pass to *map* you would have a List of NodeSeq. Flatmap means
>> you'll get the NodeSeq for each List element concatenated into one long
>> NodeSeq.
>> So your code needs to continue
>> ...flatmap { curExpense =>
>> bind("prefix", expenseTemplate, ...)
>>
>> }
>>
>> -------------------------------------
>>
>> pabraham<[email protected]> wrote:
>>
>> Hi Adam,
>>
>> Thanks for your advice. Unfortunately if I paste your code, I get
>> "error: not found: value expensesformonth".
>>
>> I need to convert my "m" value to an instance of Month where the ID of
>> this instance is "m", but I can't work out how.
>>
>> I then tried this:
>>
>> def allexpenses(expenseTemplate: NodeSeq): NodeSeq = Expense.findAll(By
>> (Expense.month,m.toLong)).flatmap
>>
>> but the findAll() gives a List[] not a flatmap, and my Scala isn't
>> good enough to convert from one to the other!
>>
>> Regards,
>>
>> Paul.
>>
>> On 18 Jan, 08:32, Adam Warski <[email protected]> wrote:
>>
>>> Hello,
>>
>>>> I have HTML to print out expenses for a given month (i.e. URL = .../
>>>> month/12):
>>
>>>> <lift:MonthPage.summary>
>>>> ...
>>>> <thismonth:expenseitem>
>>>> <tr>
>>>> <td><month:item /></td>
>>>> <td align="right"><month:amount /></td>
>>>> </tr>
>>>> </thismonth:expenseitem>
>>>> ...
>>>> </lift:MonthPage.summary>
>>
>>>> The area I'm having problems with is the code to drive the above
>>>> HTML. I need something like:
>>
>>>> class MonthPage {
>>>> def summary( xhtml : NodeSeq ) : NodeSeq = S.param("month") match {
>>>> case Full(m) => {
>>>> val allexpenses : NodeSeq = expensesformonth.flatmap({ month =>
>>>> bind("month", chooseTemplate("thismonth", "expenseitem",
>>>> xhtml),
>>>> "name" -> Text( month.item ),
>>>> "amount" -> Text( month.amount )
>>>> )
>>>> })
>>>> bind("thismonth", xhtml, "expenseitem" -> allexpenses)
>>>> }
>>>> case _ => {
>>>> Text( "Not a valid month." )
>>>> }
>>>> }
>>>> }
>>
>>> I think you're almost there.
>>> Try making the "allexpenses" val a function (def) NodeSeq => NodeSeq. The
>>> function takes the "template" of the month and returns it with bound
>>> values. So something like:
>>
>>> class MonthPage {
>>> def summary( xhtml : NodeSeq ) : NodeSeq = S.param("month") match {
>>> case Full(m) => {
>>> def allexpenses(expenseTemplate: NodeSeq): NodeSeq =
>>> expensesformonth.flatmap({ month =>
>>> bind("month", expenseTemplate,
>>> "name" -> Text( month.item ),
>>> "amount" -> Text( month.amount )
>>> )
>>> })
>>> bind("thismonth", xhtml, "expenseitem" -> allexpenses _)
>>> }
>>> case _ => {
>>> Text( "Not a valid month." )
>>> }
>>> }
>>
>>> }
>>
>>> No need to lookup the right template then using chooseTemplate, as the
>>> method receives the appropriate xhtml.
>>
>>> --
>>> Adam
>>
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