Wednesday, 16 March 2016

Building reusable, type-safe Twirl components

I've been doing quite a lot of work on a Play Framework 2.4.x app recently, a hit upon a little problem that others have noted as well. I'm trying to make the view layer as nice a place to be as the "main" codebase - after all, it's all Scala - and so I'm extracting out anything re-usable into a components package.

Here's a simple example. I'm using Bootstrap (of course), and I'm using the table-striped class to add a little bit of interest to tabular data. The setup of an HTML table is quite verbose and definitely doesn't need to be repeated, so I started with the following basic structure:
@(items:Seq[_], headings:Seq[String] = Nil)
  <table class="table table-striped">
      @if(headings.nonEmpty) {
      <thead>
          <tr>
            @for(heading <- headings) {
                <th>@heading</th>
            }
          </tr>
      </thead>
      }
      <tbody>
        @for(item <- items) {
            <tr>
                ???
            </tr>
          }
        }
      </tbody>
  </table>
Which neatens up the call-site from 20-odd lines to one:
  @stripedtable(userList, Seq("Name", "Age")

Except. How do I render each row in the table body? That differs for every use case!
What I really wanted was to be able to map over each of the items, applying some client-provided function to render a load of <td>...</td> cells for each one. Basically, I wanted stripedtable to have this signature:
@(items:Seq[T], headings:Seq[String] = Nil)(fn: T => Html)
With the body simply being:
   @for(item <- items) {
      <tr>
        @fn(item)
      </tr>
   }
and client code looking like this:
  @stripedtable(userList, Seq("Name", "Age") { user:User =>
    <td>@user.name</td><td>@user.age</td>
  }
...aaaaand we have a big problem. At least at time of writing, Twirl templates cannot be given type arguments. So those [T]'s just won't work. Loosening off the types like this:
@(items:Seq[_], headings:Seq[String] = Nil)(fn: Any => Html)
will compile, but the call-site won't work because the compiler has no idea that the _ and the Any are referring to the same type. Workaround solutions? There are two, depending on how explosively you want type mismatches to fail:
Option 1: Supply a case as the row renderer
  @stripedtable(userList, Seq("Name", "Age") { case user:User =>
    <td>@user.name</td><td>@user.age</td>
  }
This works fine, as long as every item in userList is in fact a User - if not, you get a big fat MatchError.
Option 2: Supply a case as the row renderer, and accept a PartialFunction
The template signature becomes:
@(items:Seq[_],hdgs:Seq[String] = Nil)(f: PartialFunction[Any, Html])
and we tweak the body slightly:
   @for(item <- items) {
      @if(fn.isDefinedAt(item)) {
        <tr>
          @fn(item)
        </tr>
      }
   }
In this scenario, we've protected ourselves against type mismatches, and simply skip anything that's not what we expect. Either way, I can't currently conceive of a more succinct, reusable, and obvious way to drop a consistently-built, styled table into a page than this:
  @stripedtable(userList, Seq("Name", "Age") { case user:User =>
    <td>@user.name</td><td>@user.age</td>
  }

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