Scala idioms

Introducing Scala Idioms

Rishi Khandelwal
Software Consultant
Knoldus Software LLP
Email : rishi@knoldus.com

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Using null in Scala code is not idiomatic at all
private var author:Author = null (Don't Do)
If you can’t give a default value, you really should use
var author: Option[Author] = None

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Use short names for small scopes :
is, js and ks are all but expected in loops.

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Use longer names for larger scopes :
External APIs should have longer and explanatory names that confer
meaning. Future.collect not Future.all.

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Use common abbreviations :
ex : ok, err

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Don't rebind names for different uses :
Use vals

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Avoid using `s to overload reserved names :
typ instead of `type`

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Use active names for operations with side effects :
user.activate() not user.setActive()

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Use descriptive names for methods that return values :
file.isExist not file.exist

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Don't prefix getters with get
money.count not money.getCount

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Don't repeat names that are already encapsulated in package or object
name
Prefer:
object User {
def get(id: Int): Option[User]
}
to
object User {
def getUser(id: Int): Option[User]
}

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Sort import lines alphabetically. This makes it easy to examine visually, and
is simple to automate.
Use braces when importing several names from a package :
import com.twitter.concurrent.{Broker, Offer}

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Use wildcards when more than six names are imported :
import com.twitter.concurrent._

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When using collections, qualify names by importing
scala.collection.immutable and/or scala.collection.mutable
e.g. "immutable.Map"

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Do not use relative imports from other packages :
Avoid
import com.twitter
import concurrent
in favor of the unambiguous
import com.twitter.concurrent

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Put imports at the top of the file. The reader can refer to all imports in one
place

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Use pattern matching directly in function definitions whenever applicable
instead of :
list map { item =>
item match {
case Some(x) => x
case None => default
}}
Use this :
list map {
case Some(x) => x
case None => default
}

Use the following style:
**
* ServiceBuilder builds services
* ...
*/
but not the standard ScalaDoc style:
/** ServiceBuilder builds services
* ...
*/

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Scala allows return type annotations to be omitted.
Return type is especially important for public methods.
Return type is especially important when instantiating objects with mixins
Instead: trait Service
def make() = new Service {
def getId = 123
}
Use : def make(): Service = new Service{}

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Use the mutable namespace explicitly. Don’t import
scala.collection.mutable._ and refer to Set,
use this :
import scala.collection.mutable
val set = mutable.Set()

Use the default constructor for the collection type

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val seq = Seq(1, 2, 3)
val set = Set(1, 2, 3)
val map = Map(1 -> "one", 2 -> "two", 3 -> "three")

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It is often appropriate to use lower level collections in situations that require
better performance or space efficiency. Use arrays instead of lists for large
sequences (the immutable Vector collections provides a referentially
transparent interface to arrays); and use buffers instead of direct sequence
construction when performance matters.

for (item <- container) {

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if (item != 2) return
}
It is often preferrable to call foreach, flatMap, map, and filter directly — but
do use fors when they clarify.

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Querying and transforming data: map, flatMap, filter and fold
List(1, 2, 3).map(_ * 2)
.filter(_ > 2)
.foldLeft(0)(_ + _) //> res1: Int = 10

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Pattern matching works best when also combined with destructuring
instead of
animal match {
case dog: Dog => "dog (%s)".format(dog.breed)
case _ => animal.species
}
write
animal match {
case Dog(breed) => "dog (%s)".format(breed)
case other => other.species
}

Don’t use pattern matching for conditional execution when defaults make more sense.
avoid
val x = list match {
case head :: _ => head
case Nil => default
}
use
val x = list.headOption getOrElse default
Traits that contain implementation are not directly usable from Java: extend an abstract class with the
trait instead.
// Not directly usable from Java
trait Animal {
def eat(other: Animal)
def eatMany(animals: Seq[Animal) = animals foreach(eat(_))
}
// But this is:
abstract class JavaAnimal extends Animal

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Use private[this] = visibility to the particular instance. Sometimes it aid performance
optimizations.

In singleton class types , visibility can be constrained by declaring the returned type:
def foo(): Foo with Bar = new Foo with Bar with Baz {
...
}

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Do not use structural types in normal use.
private type FooParam = {
val baz: List[String => String]
def bar(a: Int, b: Int): String
}
def foo(a: FooParam) = ...

Keep traits short and orthogonal: don’t lump separable functionality into a trait, think of the
smallest related ideas that fit together.
For example, imagine you have an something that can do IO:
trait IOer {
def write(bytes: Array[Byte])
def read(n: Int): Array[Byte]
}
separate the two behaviors:
trait Reader {
def read(n: Int): Array[Byte]
}
trait Writer {
def write(bytes: Array[Byte])
}
and mix them together to form what was an IOer:
new Reader with Writer…

Fluent” method chaining : You will often see Scala code that looks like this:

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people.sortBy(_.name)
.zipWithIndex
.map { case (person, i) => "%d: %s".format(i + 1, person.name) }
.foreach { println(_) }

Scala provides an exception facility, but do not use it for commonplace errors,

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Instead, encode such errors explicitly: using Option
Instead :
trait Repository[Key, Value] {
def get(key: Key): Value
}
Use :
trait Repository[Key, Value] {
def get(key: Key): Option[Value]
}

Some exceptions are fatal and should never be caught; the code
try {
operation()
} catch {
case _ => ...
} // almost always wrong, as it would catch fatal errors that need to be
propagated.
Instead :
use the com.twitter.util.NonFatal extractor to handle only nonfatal exceptions.
try {
operation()
} catch {
case NonFatal(exc) => ...
}

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Use implicits in the following situations:

a) Extending or adding a Scala-style collection
b) Adapting or extending an object (“pimp my library” pattern)
c) Use to enhance type safety by providing constraint evidence
d) To provide type evidence (typeclassing)
e) For Manifests
Do not use implicits to do automatic conversions between similar datatypes (for
example, converting a list to a stream); these are better done explicitly because the
types have different semantics, and the reader should beware of these implications.

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Phrasing your problem in recursive terms often simplifies it, and if the tail call
optimization applies (which can be checked by the @tailrec annotation), the compiler
will even translate your code into a regular loop.

Type aliases :
Use type aliases when they provide convenient naming or clarify purpose,
but do not alias types that are self-explanatory.
() => Int
is clearer than
type IntMaker = () => Int
IntMaker
since it is both short and uses a common type. However
class ConcurrentPool[K, V] {
type Queue = ConcurrentLinkedQueue[V]
type Map = ConcurrentHashMap[K, Queue]
...
}
is helpful since it communicates purpose and enhances brevity.

Don’t use subclassing when an alias will do.
trait SocketFactory extends (SocketAddress => Socket)
a SocketFactory is a function that produces a Socket.
Using a type alias
type SocketFactory = SocketAddress => Socket
is better. We may now provide function literals for values of type
SocketFactory and also use function composition:
val addrToInet: SocketAddress => Long
val inetToSocket: Long => Socket
val factory: SocketFactory = addrToInet andThen inetToSocket

Returns can be used to cut down on branching and establish
invariants.This is especially useful in “guard” clauses:
def compare(a: AnyRef, b: AnyRef): Int = {
if (a eq b)
return 0
val d = System.identityHashCode(a) compare
System.identityHashCode(b)
if (d != 0)
return d
// slow path..
}

Use returns to clarify and enhance readability, but not as you would in an
imperative language; avoid using them to return the results of a computation.
Instead of
def suffix(i: Int) = {
if

(i == 1) return "st"

else if (i == 2) return "nd"
else if (i == 3) return "rd"
else

return "th"

}
prefer:
def suffix(i: Int) =
if

(i == 1) "st"

else if (i == 2) "nd"
else if (i == 3) "rd"
else

"th"

require and assert both serve as executable documentation. Both are useful for
situations in which the type system cannot express the
required invariants. assert is used for invariants that the code assumes (either internal
or external), for example
val stream = getClass.getResourceAsStream("someclassdata")
assert(stream != null)

Whereas require is used to express API contracts:
def fib(n: Int) = {
require(n > 0)
...
}

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