What You Need to Know About Lambdas

Jamie Allen | Typesafe

What You Need to Know About Lambdas

Wednesday, November 6, 13

Who Am I?
• Director of Consulting for
• User of lambdas on the JVM for ~5 years
@jamie_allen


I Love Functional Programming!

• Functional Programming is:
• Immutability
• Referential Transparency
• Functions as ﬁrst-class citizens

We Want Declarative Code
final List<Integer> numbers =
Arrays.asList(1, 2, 3);
final List<Integer> numbersPlusOne =
Collections.emptyList();
for (Integer number : numbers) {
final Integer numberPlusOne = number + 1;
numbersPlusOne.add(numberPlusOne);
}


What is a Lambda?
• A function literal
• Not bound to a variable name, can only be
used in the context of where it is deﬁned

• Merely one of many possible

implementations you can use in Functional
Programming


Java 8
import java.util.List;
import java.util.Arrays;
import java.util.stream.Collectors;
public class LambdaDemo {
public static void main(String... args) {
final List<Integer> numbers =
Arrays.asList(1, 2, 3);

}

}


λ

numbers.stream().map(number -> number + 1).
collect(Collectors.toList());

Nashorn Javascript
#!/usr/bin/env jjs -scripting
var result = [];
var list = new java.util.ArrayList();
list.add(1);
list.add(2);
list.add(3);
list.parallelStream().
map(function(e) e + 1).
forEach(function(t) result.push(t));


λ

Scala
λ
object LambdaDemo extends App {
val numbers = List(1, 2, 3)
val numbersPlusOne =
numbers.map(number => number + 1)
}


Clojure
λ
(ns LambdaDemo.core)
(defn -main [& args]
(println(map #(+ % 1) [1, 2, 3])))


JRuby
require "java"
array = [1, 2, 3]
array.collect! do |n|
n + 1
end


λ

What is the Problem?


There Are Caveats


Not Reusable
• Lambdas are limited in scope to their call
site

• You cannot reuse the functionality
elsewhere


Not Testable in Isolation
• How can you test code by itself when you
have no identiﬁer through which you can
call it?

• You can only test them by writing more
tests for their enclosing method


Maintainability
• There is nothing inherently descriptive
about a lambda

• Developers have to read through the entire
lambda to ﬁgure out what it is doing

• The more complex the lambda is, the
harder this is to do

• Waste of valuable development time

Example
• In Scala, I sometimes see code like this:

val
next
=
x.map
{

case
Success(k)
=>
{

deriveValueAsynchronously(worker(initValue))(pec).map
{

case
None
=>
{

val
remainingWork
=
k(Input.EOF)

success(remainingWork)

None

}

case
Some(read)
=>
{

val
nextWork
=
k(Input.El(read))

Some(nextWork)

}

}(dec)

}

case
_
=>
{
success(it);
Future.successful(None)
}

}(dec)


}}

}λ
}λ
}

λ

λ

λ

Lousy Stack Traces
• Compilers have to come up with generic

names for their representation of lambdas
to run on the JVM, called “name mangling”

• The stack trace output tells you very little
about where the problem occurred


Java 8
numbers.stream().map(number -> number / 0)
Exception in thread "main" java.lang.ArithmeticException: / by zero
at LambdaDemo.lambda$0(LambdaDemo.java:9)
at LambdaDemo$$Lambda$1.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:510)
at LambdaDemo.main(LambdaDemo.java:9)

wat


Nashorn Javascript
list.parallelStream().
map(function(e) e / 0)
[1, 2, 3]
Infinity,Infinity


Favorite Tweet Ever
“JavaScript doesn't have a dark side, but
it does have a dimly lit room full of
angry clowns with rubber mallets.”
- @odetocode, Jan 5, 2010


Scala
val numbersPlusOne = numbers.map(number => number / 0)
at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$$anonfun$1.apply(LambdaPlayground.scala:23)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.immutable.Range.foreach(Range.scala:141)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
at scala.collection.AbstractTraversable.map(Traversable.scala:105)
at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$delayedInit$body.apply(LambdaPlayground.scala:23)
at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
at scala.App$$anonfun$main$1.apply(App.scala:71)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
at scala.App$class.main(App.scala:71)
at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$.main(LambdaPlayground.scala:22)
at org.jamieallen.effectiveakka.pattern.extra.LambdaTest.main(LambdaPlayground.scala)

wat


Clojure

println(map #(/ % 0) [1, 2, 3])))

Exception in thread "main" (java.lang.ArithmeticException: Divide by zero
at clojure.lang.Numbers.divide(Numbers.java:156)
at clojure.lang.Numbers.divide(Numbers.java:3671)
at helloclj.core$_main$fn__10.invoke(core.clj:5)
at clojure.core$map$fn__4087.invoke(core.clj:2432)
at clojure.lang.LazySeq.sval(LazySeq.java:42)
at clojure.lang.LazySeq.seq(LazySeq.java:60)
at clojure.lang.RT.seq(RT.java:473)
at clojure.core$seq.invoke(core.clj:133)
at clojure.core$print_sequential.invoke(core_print.clj:46)
at clojure.core$fn__5270.invoke(core_print.clj:140)
at clojure.lang.MultiFn.invoke(MultiFn.java:167)
at clojure.core$pr_on.invoke(core.clj:3266)
at clojure.core$pr.invoke(core.clj:3278)
at clojure.lang.AFn.applyToHelper(AFn.java:161)
at clojure.lang.RestFn.applyTo(RestFn.java:132)
at clojure.core$apply.invoke(core.clj:601)
at clojure.core$prn.doInvoke(core.clj:3311)
at clojure.lang.RestFn.applyTo(RestFn.java:137)
at clojure.core$apply.invoke(core.clj:601)
at clojure.core$println.doInvoke(core.clj:3331)
at clojure.lang.RestFn.invoke(RestFn.java:408)
at helloclj.core$_main.invoke(core.clj:5)
at clojure.lang.Var.invoke(Var.java:411)
...
at clojure.main.main(main.java:37)

wat


JRuby
array.collect! do |n|
n / 0
ZeroDivisionError: divided by 0
/ at org/jruby/RubyFixnum.java:547
(root) at HelloWorld.rb:11
collect! at org/jruby/RubyArray.java:2385
(root) at HelloWorld.rb:10

not half bad, really


Difﬁcult Debugging
• Debuggers on the JVM can only

disambiguate code at the source line write your lambdas to leverage this

final List<Integer> numbersPlusOne = numbers.stream().
map(number -> number + 1).collect(Collectors.toList());

NO!


Digression: Lambdas versus Closures

• In the purest sense, closures are merely

lambdas that close over some state from
outside of their enclosing scope

final int x = 1;
numbers.stream().map(number -> number + x).
collect(Collectors.toList());


Closing Over State

•

Lambdas have access to all variables that are in
scope

•

It is very easy to “close over” something
mutable and cause headaches in multi-threaded
code

•

Java enforces that values to be closed over are
ﬁnal, but that only affects assignment - you can
still change what is INSIDE that variable (like
the contents of a collection)


Solution
We want to maintain our ability to program
in a functional style, while having something
maintainable and understandable in
production


Named Functions?
• Seems like it would help, but it depends on
the compiler and how it manages the
“scope” of that function

• It is possible that stack traces will still not
show the name of the function


Named Function
object LambdaTest extends App {
val addOneToValue = (x: Int) => x + 1
}

val myList = (1 to 20).map(addOneToValue)


Named Function Stack Trace
val badFunction = (x: Int) => x / 0
val myList = (1 to 20).map(badFunction)

at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$$anonfun$1.apply$mcII$sp(LambdaPlayground.scala:23)


















wat


“Lifting” a Method
• We have the ability in Java 8 and Scala to

“lift” or coerce a method into a function

• The method must meet the contract of the
lambda usage of the compiler, such as only
taking one argument representing the input
of the function


Stack Trace of a Method
def badFunction = (x: Int) => x / 0

at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$$anonfun$badFunction$1.apply$mcII$sp(LambdaPlayground.scala:23)


















Better, but
why $1


Digression
• You can deﬁne your methods like that, but
“def” is not stable - it will reevaluate the
right side of the equals sign and return a
new but identical function each time!
def badFunction = (x: Int) => x / 0

•

Better to stick with simple method syntax instead
def badFunction(x: Int) = x / 0


Stack Trace of a Stable Method
def badFunction(x: Int) = x / 0

at org.jamieallen.effectiveakka.pattern.extra.LambdaTest$.badFunction(LambdaPlayground.scala:24)


















Perfect!


Beneﬁts
• You can’t close over variables
• Better stack traces
• More debuggable
• More testable
• More maintainable and descriptive
• Reusable

Rule of Thumb
• Reserve lambda usage for the most basic
expressions

• Externalize anything more signiﬁcant than
that to methods


Language Creators
• Language designers and tool producers
need to help us

• At Typesafe, we’re making our Eclipse-based
Scala IDE more lambda-friendly with each
release


Thank You!

• Questions?


What You Need to Know About Lambdas

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to What You Need to Know About Lambdas

Similar to What You Need to Know About Lambdas (20)

More from jaxLondonConference

More from jaxLondonConference (20)

Recently uploaded

Recently uploaded (20)

What You Need to Know About Lambdas