Nested String Interpolation in Scala

Question

I'm executing a query on RDBMS and getting the result as a String. The String looks something like this:

val DBASE = "my_database"
val FREQ = "monthly"
val queryResult: String = sqlContext.read.jdbc(...).collect.map(...).first
// queryResult = Database is $$${DBASE} and frequency is $$${FREQ}

Next I'm replacing $$$ with $, so I'm left with something like:

queryResult = "Database is ${DBASE} and frequency is ${FREQ}"

How to use String Interpolation on DBASE and FREQ? I have tried:

val substituted = f"${queryResult}"

and

val substituted = s"${queryResult}"

But the variables are not getting replaced. Is there something else I can do apart from using the following :

queryResult.replaceAll("\\$\\{DBASE\\}", DBASE).replaceAll(...)

som-snytt · Accepted Answer · 2018-07-14 01:41:24Z

One option is to let scripting do your templating:

     ________ ___   / /  ___  
    / __/ __// _ | / /  / _ | 
  __\ \/ /__/ __ |/ /__/ __ | 
 /____/\___/_/ |_/____/_/ | | 
                          |/  version 2.12.6

scala> import javax.script._
import javax.script._

scala> val se = new ScriptEngineManager().getEngineByName("scala")
se: javax.script.ScriptEngine = scala.tools.nsc.interpreter.Scripted@6549ce71

scala> val b = se.createBindings
b: javax.script.Bindings = javax.script.SimpleBindings@2648aa1b

scala> b.put("DBASE", "my_db")
res0: Object = null

scala> b.put("FREQ", "monthly")
res1: Object = null

scala> se.eval("""s"Db is $DBASE, freq is $FREQ"""", b)
res2: Object = Db is my_db, freq is monthly

The bound objects aren't well-typed, but you can cast or what have you and do more computation:

scala> se.eval("""s"Db is $DBASE, freq is ${FREQ.toString * 2}"""", b)
res4: Object = Db is my_db, freq is monthlymonthly

Andrey Tyukin · Accepted Answer · 2018-07-13 14:14:17Z

2

Scala's string interpolation

s"Database is ${DBASE} and frequency is ${FREQ}"

is essentially just fancy syntax for a method call on StringContext. It desugars to something like

StringContext("Database is ", " and frequency is ",  "").s(DBASE, FREQ)

While you could theoretically split the string "Database is ${DBASE} and frequency is ${FREQ}" into pieces that are appropriate for StringContext, and then look up the variables DBASE and FREQ somewhere, it seems that it is much easier to replaceAll occurrences of $$${DBASE} and $$${FREQ} right away.

answered Jul 13, 2018 at 14:14

Andrey Tyukin

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3 Comments

philantrovert Over a year ago

It's just that string is almost 50,000 characters ( a parameterized SQL query that I need to execute ) so I don't think it'll be very feasible to split and use StringContext. Guess I'll use replaceAll then.

Andrey Tyukin Over a year ago

@philantrovert I'm not sure why you are trying to manually plug in parameters into queries that are returned by other queries... Seems like a strange stringly-typed design. Is there no way to use some kind of prepared statement instead?

philantrovert Over a year ago

No, the query needs to be passed to hive.

Chaitanya · Accepted Answer · 2018-07-13 14:16:01Z

0

I think it is not possible dynamically to interpolate a string as it happens during compile time itself.

1) String interpolation happens at compile time, so the compiler does not generally have enough information to interpolate s(queryResult). It expects a string literal, according to https://docs.scala-lang.org/sips/string-interpolation.html.

2) Under Advanced Usage in the documentation, it is explained that an expression of the form id"Hello $name ." is translated at compile time to new StringContext("Hello", "."). id(name).

So you can create something like

StringContext("Database is ", " and frequency is ",  "").s(DBASE, FREQ)

which will substitute the syntactic suger. Hope that helps.

answered Jul 13, 2018 at 14:16

Chaitanya

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