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jsonscalaplayframeworkplay-json

Dealing with JSON in Scala?

In Scala 2.11, having the below code:

import play.api.libs.json._
...
val data = // read json from file                             (3)
val JSON: JsValue = Json.parse(data mkString "\n")            (4)
val items = JSON \ "items"
for (i <- 0 until 100) yield items(i)
  1. if I unite the last two lines for (i <- 0 until 100) yield (JSON \ "items")(i), will the term JSON \ "items" be evaluated for each i or only once?
  2. is it worth to parallelise the list construction with this for-expression (I don't care about the order in which items will appear in the list), where items is an array of JSON objects?
  3. what is the best way to handle exceptions from parsing the JSON in the lines (3 - 4) and validate it?
Solution

    1. If you use the expression JSON \ "items" 100 times instead of 1, there'll be 100 times the work to find those nodes - there's no majick memoization or anything like that going on. Your cost is O(n) relative to the number of times you execute it - not O(1). But in any case, for this application the difference is inconsequential - assuming there's no outer loop you're not showing us.

    2. This is way too small beans for parallelization to make any sense - in fact, the overhead might slow things down. If your real case was yield expensiveComputationBasedOn(items(i)), then maybe.

    3. For lines 3-4, yes, use a Try here if you need to handle it here, else farther up (in the method that called the method that called this). In general, catch exceptions at the highest level where you can still provide sufficient information about what went wrong in a log message, where you can do any failure recovery, and where you'll be able to debug. This saves work and makes sure that you catch everything - even what you don't think of. If that's in your "main", fine. An Option will not catch exceptions. Caution: if this is for a class, your teacher may be looking for local error handling, regardless.