How to profile methods in Scala?

Do you want to do this without changing the code that you want to measure timings for? If you don't mind changing the code, then you could do something like this:

def time[R](block: => R): R = {
    val t0 = System.nanoTime()
    val result = block    // call-by-name
    val t1 = System.nanoTime()
    println("Elapsed time: " + (t1 - t0) + "ns")
    result
}

// Now wrap your method calls, for example change this...
val result = 1 to 1000 sum

// ... into this
val result = time { 1 to 1000 sum }

This what I use:

import System.nanoTime
def profile[R](code: => R, t: Long = nanoTime) = (code, nanoTime - t)

// usage:
val (result, time) = profile { 
  /* block of code to be profiled*/ 
}

val (result2, time2) = profile methodToBeProfiled(foo)

In addition to Jesper's answer, you can automatically wrap method invocations in the REPL:

scala> def time[R](block: => R): R = {
   | val t0 = System.nanoTime()
   | val result = block
   | println("Elapsed time: " + (System.nanoTime - t0) + "ns")
   | result
   | }
time: [R](block: => R)R

Now - let's wrap anything in this

scala> :wrap time
wrap: no such command.  Type :help for help.

OK - we need to be in power mode

scala> :power
** Power User mode enabled - BEEP BOOP SPIZ **
** :phase has been set to 'typer'.          **
** scala.tools.nsc._ has been imported      **
** global._ and definitions._ also imported **
** Try  :help,  vals.<tab>,  power.<tab>    **

Wrap away

scala> :wrap time
Set wrapper to 'time'

scala> BigDecimal("1.456")
Elapsed time: 950874ns
Elapsed time: 870589ns
Elapsed time: 902654ns
Elapsed time: 898372ns
Elapsed time: 1690250ns
res0: scala.math.BigDecimal = 1.456

I have no idea why that printed stuff out 5 times

Update as of 2.12.2:

scala> :pa
// Entering paste mode (ctrl-D to finish)

package wrappers { object wrap { def apply[A](a: => A): A = { println("running...") ; a } }}

// Exiting paste mode, now interpreting.


scala> $intp.setExecutionWrapper("wrappers.wrap")

scala> 42
running...
res2: Int = 42