Play/Scala Template Block Statement HTML Output Sy

Question

Ok, I've been stuggling with this one for a while, and have spent a lot of time trying different things to do something that I have done very easily using PHP.

I am trying to iterate over a list while keeping track of a variable locally, while spitting out HTML attempting to populate a table.

Attempt #1:

@{
    var curDate : Date = null
    for(ind <- indicators){
        if(curDate == null || !curDate.equals(ind.getFirstFound())){
            curDate = ind.getFirstFound()
            <tr><th colspan='5' class='day'>@(ind.getFirstFound())</th></tr>
            <tr><th>Document ID</th><th>Value</th><th>Owner</th><th>Document Title / Comment</th></tr>
        }
    }
}

I attempt too user a scala block statement to allow me to keep curDate as a variable within the created scope. This block correctly maintains curDate state, but does not allow me to output anything to the DOM. I did not actually expect this to compile, due to my unescaped, randomly thrown in HTML, but it does. this loop simply places nothing on the DOM, although the decision structure is correctly executed on the server.

I tried escaping using @Html('...'), but that produced compile errors.

Attempt #2:

A lot of google searches led me to the "for comprehension":

@for(ind <- indicators; curDate = ind.getFirstFound()){
    @if(curDate == null || !curDate.equals(ind.getFirstFound())){
        @(curDate = ind.getFirstFound())
    }
    <tr><th colspan='5' class='day'>@(ind.getFirstFound())</th></tr>
    <tr><th>Document ID</th><th>Value</th><th>Owner</th><th>Document Title / Comment</th></tr>
}

Without the if statement in this block, this is the closest I got to doing what I actually wanted, but apparently I am not allowed to reassign a non-reference type, which is why I was hoping attempt #1's reference declaration of curDate : Date = null would work. This attempt gets me the HTML on the page (again, if i remove the nested if statement) but doesn't get me the

My question is, how do i implement this intention? I am very painfully aware of my lack of Scala knowledge, which is being exacerbated by Play templating syntax. I am not sure what to do.

Thanks in advance!

Answer 1

Play's template language is very geared towards functional programming. It might be possible to achieve what you want to achieve using mutable state, but you'll probably be best going with the flow, and using a functional solution.

If you want to maintain state between iterations of a loop in functional programming, that can be done by doing a fold - you start with some state, and on each iteration, you get the previous state and the next element, and you then return the new state based on those two things.

So, looking at your first solution, it looks like what you're trying to do is only print an element out if it's date is different from the previous one, is that correct? Another way of putting this is you want to filter out all the elements that have a date that's the same date as the previous one. Expressing that in terms of a fold, we're going to fold the elements into a sequence (our initial state), and if the last element of the folded sequence has a different date to the current one, we add it, otherwise we ignore it.

Our fold looks like this:

indicators.foldLeft(Vector.empty[Indicator]) { (collected, next) =>
  if (collected.lastOption.forall(_.getFirstFound != next.getFirstFound)) {
    collected :+ next
  } else {
    collected
  }
}

Just to explain the above, we're folding into a Vector because Vector has constant time append and last, List has n time. The forall will return true if there is no last element in collected, otherwise if there is, it will return true if the passed in lambda evaluates to true. And in Scala, == invokes .equals (after doing a null check), so you don't need to use .equals in Scala.

So, putting this in a template:

@for(ind <- indicators.foldLeft(Vector.empty[Indicator]) { (collected, next) =>
  if (collected.lastOption.forall(_.getFirstFound != next.getFirstFound)) {
    collected :+ next
  } else {
    collected
  }
}){
  ...
}