What are the precise rules for when you can omit (omit) parentheses, dots, braces, = (functions), etc.?
For example,
(service.findAllPresentations.get.first.votes.size) must be equalTo(2).
service is my object
def findAllPresentations: Option[List[Presentation]]
votes returns List[Vote]
must and be are both functions of specs
Why can't I go:
(service findAllPresentations get first votes size) must be equalTo(2)
?
The compiler error is:
"RestServicesSpecTest.this.service.findAllPresentations
of type
Option[List[com.sharca.Presentation]]
does not take parameters"
Why does it think I'm trying to pass in a parameter? Why must I use dots for every method call?
Why must (service.findAllPresentations get first votes size) be equalTo(2) result in:
"not found: value first"
Yet, the "must be equalTo 2" of
(service.findAllPresentations.get.first.votes.size) must be equalTo 2, that is, method chaining works fine? - object chain chain chain param.
I've looked through the Scala book and website and can't really find a comprehensive explanation.
Is it in fact, as Rob H explains in Stack Overflow question Which characters can I omit in Scala?, that the only valid use-case for omitting the '.' is for "operand operator operand" style operations, and not for method chaining?
You seem to have stumbled upon the answer. Anyway, I'll try to make it clear.
You can omit dot when using the prefix, infix and postfix notations -- the so called operator notation. While using the operator notation, and only then, you can omit the parenthesis if there is less than two parameters passed to the method.
Now, the operator notation is a notation for method-call, which means it can't be used in the absence of the object which is being called.
I'll briefly detail the notations.
Prefix:
Only ~, !, + and - can be used in prefix notation. This is the notation you are using when you write !flag or val liability = -debt.
Infix:
That's the notation where the method appears between an object and it's parameters. The arithmetic operators all fit here.
Postfix (also suffix):
That notation is used when the method follows an object and receives no parameters. For example, you can write list tail, and that's postfix notation.
You can chain infix notation calls without problem, as long as no method is curried. For example, I like to use the following style:
(list
filter (...)
map (...)
mkString ", "
)
That's the same thing as:
list filter (...) map (...) mkString ", "
Now, why am I using parenthesis here, if filter and map take a single parameter? It's because I'm passing anonymous functions to them. I can't mix anonymous functions definitions with infix style because I need a boundary for the end of my anonymous function. Also, the parameter definition of the anonymous function might be interpreted as the last parameter to the infix method.
You can use infix with multiple parameters:
string substring (start, end) map (_ toInt) mkString ("<", ", ", ">")
Curried functions are hard to use with infix notation. The folding functions are a clear example of that:
(0 /: list) ((cnt, string) => cnt + string.size)
(list foldLeft 0) ((cnt, string) => cnt + string.size)
You need to use parenthesis outside the infix call. I'm not sure the exact rules at play here.
Now, let's talk about postfix. Postfix can be hard to use, because it can never be used anywhere except the end of an expression. For example, you can't do the following:
list tail map (...)
Because tail does not appear at the end of the expression. You can't do this either:
list tail length
You could use infix notation by using parenthesis to mark end of expressions:
(list tail) map (...)
(list tail) length
Note that postfix notation is discouraged because it may be unsafe.
I hope this has cleared all the doubts. If not, just drop a comment and I'll see what I can do to improve it.
Class definitions:
val or var can be omitted from class parameters which will make the parameter private.
Adding var or val will cause it to be public (that is, method accessors and mutators are generated).
{} can be omitted if the class has no body, that is,
class EmptyClass
Class instantiation:
Generic parameters can be omitted if they can be inferred by the compiler. However note, if your types don't match, then the type parameter is always infered so that it matches. So without specifying the type, you may not get what you expect - that is, given
class D[T](val x:T, val y:T);
This will give you a type error (Int found, expected String)
var zz = new D[String]("Hi1", 1) // type error
Whereas this works fine:
var z = new D("Hi1", 1)
== D{def x: Any; def y: Any}
Because the type parameter, T, is inferred as the least common supertype of the two - Any.
Function definitions:
= can be dropped if the function returns Unit (nothing).
{} for the function body can be dropped if the function is a single statement, but only if the statement returns a value (you need the = sign), that is,
def returnAString = "Hi!"
but this doesn't work:
def returnAString "Hi!" // Compile error - '=' expected but string literal found."
The return type of the function can be omitted if it can be inferred (a recursive method must have its return type specified).
() can be dropped if the function doesn't take any arguments, that is,
def endOfString {
return "myDog".substring(2,1)
}
which by convention is reserved for methods which have no side effects - more on that later.
() isn't actually dropped per se when defining a pass by name paramenter, but it is actually a quite semantically different notation, that is,
def myOp(passByNameString: => String)
Says myOp takes a pass-by-name parameter, which results in a String (that is, it can be a code block which returns a string) as opposed to function parameters,
def myOp(functionParam: () => String)
which says myOp takes a function which has zero parameters and returns a String.
(Mind you, pass-by-name parameters get compiled into functions; it just makes the syntax nicer.)
() can be dropped in the function parameter definition if the function only takes one argument, for example:
def myOp2(passByNameString:(Int) => String) { .. } // - You can drop the ()
def myOp2(passByNameString:Int => String) { .. }
But if it takes more than one argument, you must include the ():
def myOp2(passByNameString:(Int, String) => String) { .. }
Statements:
. can be dropped to use operator notation, which can only be used for infix operators (operators of methods that take arguments). See Daniel's answer for more information.
. can also be dropped for postfix functions
list tail
() can be dropped for postfix operators
list.tail
() cannot be used with methods defined as:
def aMethod = "hi!" // Missing () on method definition
aMethod // Works
aMethod() // Compile error when calling method
Because this notation is reserved by convention for methods that have no side effects, like List#tail (that is, the invocation of a function with no side effects means that the function has no observable effect, except for its return value).
() can be dropped for operator notation when passing in a single argument
() may be required to use postfix operators which aren't at the end of a statement
() may be required to designate nested statements, ends of anonymous functions or for operators which take more than one parameter
When calling a function which takes a function, you cannot omit the () from the inner function definition, for example:
def myOp3(paramFunc0:() => String) {
println(paramFunc0)
}
myOp3(() => "myop3") // Works
myOp3(=> "myop3") // Doesn't work
When calling a function that takes a by-name parameter, you cannot specify the argument as a parameter-less anonymous function. For example, given:
def myOp2(passByNameString:Int => String) {
println(passByNameString)
}
You must call it as:
myOp("myop3")
or
myOp({
val source = sourceProvider.source
val p = myObject.findNameFromSource(source)
p
})
but not:
myOp(() => "myop3") // Doesn't work
IMO, overuse of dropping return types can be harmful for code to be re-used. Just look at specification for a good example of reduced readability due to lack of explicit information in the code. The number of levels of indirection to actually figure out what the type of a variable is can be nuts. Hopefully better tools can avert this problem and keep our code concise.
(OK, in the quest to compile a more complete, concise answer (if I've missed anything, or gotten something wrong/inaccurate please comment), I have added to the beginning of the answer. Please note this isn't a language specification, so I'm not trying to make it exactly academically correct - just more like a reference card.)
A collection of quotes giving insight into the various conditions...
Personally, I thought there'd be more in the specification. I'm sure there must be, I'm just not searching for the right words...
There are a couple of sources however, and I've collected them together, but nothing really complete / comprehensive / understandable / that explains the above problems to me...:
"If a method body has more than one
expression, you must surround it with
curly braces {…}. You can omit the
braces if the method body has just one
expression."
From chapter 2, "Type Less, Do More", of Programming Scala:
"The body of the upper method comes
after the equals sign ‘=’. Why an
equals sign? Why not just curly braces
{…}, like in Java? Because semicolons,
function return types, method
arguments lists, and even the curly
braces are sometimes omitted, using an
equals sign prevents several possible
parsing ambiguities. Using an equals
sign also reminds us that even
functions are values in Scala, which
is consistent with Scala’s support of
functional programming, described in
more detail in Chapter 8, Functional
Programming in Scala."
From chapter 1, "Zero to Sixty: Introducing Scala", of Programming Scala:
"A function with no parameters can be
declared without parentheses, in which
case it must be called with no
parentheses. This provides support for
the Uniform Access Principle, such
that the caller does not know if the
symbol is a variable or a function
with no parameters.
The function body is preceded by "="
if it returns a value (i.e. the return
type is something other than Unit),
but the return type and the "=" can be
omitted when the type is Unit (i.e. it
looks like a procedure as opposed to a
function).
Braces around the body are not
required (if the body is a single
expression); more precisely, the body
of a function is just an expression,
and any expression with multiple parts
must be enclosed in braces (an
expression with one part may
optionally be enclosed in braces)."
"Functions with zero or one argument
can be called without the dot and
parentheses. But any expression can
have parentheses around it, so you can
omit the dot and still use
parentheses.
And since you can use braces anywhere
you can use parentheses, you can omit
the dot and put in braces, which can
contain multiple statements.
Functions with no arguments can be
called without the parentheses. For
example, the length() function on
String can be invoked as "abc".length
rather than "abc".length(). If the
function is a Scala function defined
without parentheses, then the function
must be called without parentheses.
By convention, functions with no
arguments that have side effects, such
as println, are called with
parentheses; those without side
effects are called without
parentheses."
From blog post Scala Syntax Primer:
"A procedure definition is a function
definition where the result type and
the equals sign are omitted; its
defining expression must be a block.
E.g., def f (ps) {stats} is
equivalent to def f (ps): Unit =
{stats}.
Example 4.6.3 Here is a declaration
and a de?nition of a procedure named
write:
trait Writer {
def write(str: String)
}
object Terminal extends Writer {
def write(str: String) { System.out.println(str) }
}
The code above is implicitly completed
to the following code:
trait Writer {
def write(str: String): Unit
}
object Terminal extends Writer {
def write(str: String): Unit = { System.out.println(str) }
}"
From the language specification:
"With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses, enabling the operator
syntax shown in our insertion operator
example. This syntax is used in other
places in the Scala API, such as
constructing Range instances:
val firstTen:Range = 0 to 9
Here again, to(Int) is a vanilla
method declared inside a class
(there’s actually some more implicit
type conversions here, but you get the
drift)."
From Scala for Java Refugees Part 6: Getting Over Java:
"Now, when you try "m 0", Scala
discards it being a unary operator, on
the grounds of not being a valid one
(~, !, - and +). It finds that "m" is
a valid object -- it is a function,
not a method, and all functions are
objects.
As "0" is not a valid Scala
identifier, it cannot be neither an
infix nor a postfix operator.
Therefore, Scala complains that it
expected ";" -- which would separate
two (almost) valid expressions: "m"
and "0". If you inserted it, then it
would complain that m requires either
an argument, or, failing that, a "_"
to turn it into a partially applied
function."
"I believe the operator syntax style
works only when you've got an explicit
object on the left-hand side. The
syntax is intended to let you express
"operand operator operand" style
operations in a natural way."
Which characters can I omit in Scala?
But what also confuses me is this quote:
"There needs to be an object to
receive a method call. For instance,
you cannot do “println “Hello World!”"
as the println needs an object
recipient. You can do “Console
println “Hello World!”" which
satisfies the need."
Because as far as I can see, there is an object to receive the call...
I find it easier to follow this rule of thumb: in expressions spaces alternate between methods and parameters. In your example, (service.findAllPresentations.get.first.votes.size) must be equalTo(2) parses as (service.findAllPresentations.get.first.votes.size).must(be)(equalTo(2)). Note that the parentheses around the 2 have a higher associativity than the spaces. Dots also have higher associativity, so (service.findAllPresentations.get.first.votes.size) must be.equalTo(2)would parse as (service.findAllPresentations.get.first.votes.size).must(be.equalTo(2)).
service findAllPresentations get first votes size must be equalTo 2 parses as service.findAllPresentations(get).first(votes).size(must).be(equalTo).2.
Actually, on second reading, maybe this is the key:
With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses
As mentioned on the blog post: http://www.codecommit.com/blog/scala/scala-for-java-refugees-part-6 .
So perhaps this is actually a very strict "syntax sugar" which only works where you are effectively calling a method, on an object, which takes one parameter. e.g.
1 + 2
1.+(2)
And nothing else.
This would explain my examples in the question.
But as I said, if someone could point out to be exactly where in the language spec this is specified, would be great appreciated.
Ok, some nice fellow (paulp_ from #scala) has pointed out where in the language spec this information is:
6.12.3:
Precedence and associativity of
operators determine the grouping of
parts of an expression as follows.
If there are several infix operations in an expression, then
operators with higher precedence bind
more closely than operators with lower
precedence.
If there are consecutive infix operations e0 op1 e1 op2 . . .opn en
with operators op1, . . . , opn of the
same precedence, then all these
operators must have the same
associativity. If all operators are
left-associative, the sequence is
interpreted as (. . . (e0 op1 e1) op2
. . .) opn en. Otherwise, if all
operators are rightassociative, the
sequence is interpreted as e0 op1 (e1
op2 (. . .opn en) . . .).
Postfix operators always have lower precedence than infix operators. E.g.
e1 op1 e2 op2 is always equivalent to
(e1 op1 e2) op2.
The right-hand operand of a
left-associative operator may consist
of several arguments enclosed in
parentheses, e.g. e op (e1, . . .
,en). This expression is then
interpreted as e.op(e1, . . . ,en).
A left-associative binary operation e1
op e2 is interpreted as e1.op(e2). If
op is rightassociative, the same
operation is interpreted as { val
x=e1; e2.op(x ) }, where x is a fresh
name.
Hmm - to me it doesn't mesh with what I'm seeing or I just don't understand it ;)
There aren't any. You will likely receive advice around whether or not the function has side-effects. This is bogus. The correction is to not use side-effects to the reasonable extent permitted by Scala. To the extent that it cannot, then all bets are off. All bets. Using parentheses is an element of the set "all" and is superfluous. It does not provide any value once all bets are off.
This advice is essentially an attempt at an effect system that fails (not to be confused with: is less useful than other effect systems).
Try not to side-effect. After that, accept that all bets are off. Hiding behind a de facto syntactic notation for an effect system can and does, only cause harm.
Method types have no value. How do we evaluate a method?
Using SML as an example, I have
fun myFunc(x) = x + 5
val b = myFunc(2)
In the second expression, myFun has a type and a value, we use its type to do type checking and use its value together with its argument to evaluate value for b
But in Scala methods without a value how do we evaluate? I am pretty new to Scala so it may not be very clear.
def myFunc(x) = x + 5
val b = myFunc(2)
From val b = myFunc(2) to val b = 2 + 5, what happened in between? From where or what object do we know that myFunc(x) is x + 5?
THanks!!
The simple answer is: just because a method is not a value in the sense of "a thing that can be manipulated by you" doesn't mean that it is not a value in the sense of "a thing that can be manipulated by the author of the compiler".
Of course, a method will have an object representing it inside of the compiler. In fact, that object will probably look very similar to the object representing a function inside, say, the MLTon SML compiler or SML/NJ.
In SML, syntax is not a value, but you are not questioning how it is possible to write a function call either, aren't you? After all, in order to call a function in SML, I need to write a function call using function call syntax, so how can I do that when syntax is not a value?
Well, the answer is the same: just because syntax is not a value that the programmer can manipulate, the compiler (or more precisely the parser) obviously does know about syntax.
I can't tell you why the decision was made to have functions be values in Scala but not methods, but I can make a guess. Scala is an object-oriented language. In an object-oriented language, every value is an object, and every object has methods that are bound to that object. So, if methods are objects, they need to have methods, which are objects, which have methods, which are objects, and so on.
There are ways to deal with this, of course, but it makes the language more complex. For a similar reason, classes aren't objects (unlike, say, in Smalltalk, Python, and Ruby). Note that even in highly reflective, introspective, dynamic languages like Ruby, methods are not objects. Classes are, but not methods.
It is possible using reflection to get a proxy object that represents a method, but that object is not the method itself. And you can actually do the same in Scala as well.
And of course it is possible to turn a method into a function value by η-expansion.
I'm assuming that you're compiling to Java Virtual Machine (JVM) bytecode, such as with scalac, which is probably the most common way to use Scala. Disclaimer: I'm not an expert on the JVM, so some parts of this answer might be a bit wrong, but the general idea is right.
Essentially, a method is a set of instructions for the runtime to execute. It exists as part of the compiled code on disk (e.g. a .class file). When the JVM loads the class, it pulls the entire class file into memory, including the methods. When the JVM encounters a method call, it looks up the method and starts executing the instructions in it. If the method returns a result, the JVM makes that result available in the calling code, then does whatever you wanted to do with it there, such as assigning to a variable.
With that knowledge, we can answer some of your questions:
From val b = myFunc(2) to val b = 2 + 5, what happened in between?
This isn't quite how it works, as the JVM doesn't "expand" myFunc in place, but instead looks up myFunc and executes the instructions in it.
From where or what object do we know that myFunc(x) is x + 5?
Not from any object. While myFunc is in memory, it's in an area of memory that you can't access directly (but the JVM can).
why can't it be a value since it is a chunk of memory?
Not all memory fits into the nice abstractions of types and values.
More explicitly, can this code produce any errors in any scenrios:
def foreach[U](f :Int=>U) = f.asInstanceOf[Int=>Unit](1)
I know it works, and I have a vague idea why: any function, as an instance of a generic type, must define an erased version of apply and jvm performs type check only when the object is actually to be returned to a code where it had a concrete type (often miles away). So, in theory, as long as I never look at the returned value, I should be safe. I don't have an enough low-level understandings of java byte code, let alone scalac, to have any certainty about it.
Why would I want to do it? Look at the following example:
val b = new mutable.Buffer[Int]
val ints = Seq(1, 2, 3, 4)
ints foreach { b += _ }
It's a typical scala construct, as far as imperative style can be typical. foreach in this example takes an Int as an argument, and as scalac knows it to be an Int, it will create a closure with a specialized apply(x :Int). Unfortunately, its return type in this case is a mutable.Buffer[Int], which is an AnyRef. As far as I was able to see, scalac will never invoke a specialized apply providing an AnyVal argument if the result is an AnyRef (and vice versa). This means, that even if the caller applies the function to Int, underneath the function will box the argument and invoke the erased variant. Here of course it doesn't matter as they are boxed within the List anyway, but I'm talking about the principle.
For this reason I prefer to define this type of method as foreach(f :X=>Unit), rather than foreach[O](f: X=>O) as it is in TraversableOnce. If the input sequence in the example had such a signature, everything would compile just as fine, and the compiler would ignore the actual type of the expression and generate a function with Unit return type, which - when applied to an unboxed Int - would invoke directly void apply(Int x), without boxing.
The problem arises with interoperability - sometimes I need to call a method expecting a function with a Unit return type and all I have is a generic function returning Odin knows what. Of course, I could just write f(_) to box it in another function object instead of passing it directly, but it to large extent makes the whole optimisation of small tight loops moot.
I am looking at a piece of code with the following:
graph.vertices.filter(!_._2._1)
I understand that _ are wildcard characters in scala but I do not know what the ! is supposed to do.
What does ! mean in scala?
Scala doesn't have operators at the syntax level. All operations are methods.
For example, there is no add operator in the syntax, but numbers have a + method:
2.+(3) // result is 5
When you write 2 + 3, that's actually syntax sugar for the expression above.
Any type can define a unary_! method, which is what !something gets desugared to. Booleans implement it, with the obvious meaning of logical negation ("not") that the exclamation mark has in other languages with C heritage.
In your question, the expression is an abbreviated form of the following call:
graph.vertices.filter { t => !(t._2._1) }
where t is a tuple-of-tuples, for which the first element of the second element has a type that implements unary_! and (as required by .filter) returns a Boolean. I would bet the money in my pocket that the element itself is a Boolean, in which case ! just means "not."
As Robert said, ! is a method name. It can be tricky determining which ! method is being used. For example, in the line of code:
val exitValue = command.!(ProcessLogger(stdoutWriter.println, stderrWriter.println))
where command is a String (or Seq), command can be implicitly converted to a ProcessBuilder, so its ! method would apply. Your IDE may be able to help. IntelliJ IDEA Ultimate was able to tell me where ! was defined.
If we have a method:
def hello() = "world"
I'm told that we can call it as:
hello()
also
hello
They both work and will output world, but why?
PS:
I see some words in this https://stackoverflow.com/a/12340289/342235:
No, actually, they are not. Even though they both call a method without parameters, one is a "method with zero parameter lists" while the other is a "method with one empty parameter list"
But I still not understand why hello would work
Scala allows the omission of parentheses on methods of arity-0 (no arguments):
You should only omit the parenthesis when there are no side effects to the invokation though
http://docs.scala-lang.org/style/method-invocation.html
As stated in Oliver Shaw's answer, Scala lets you leave out the parenthesis in functions with 0 arguments. As for why, it's likely to facilitate easy refactoring.
If you have a function that takes no arguments, and produces no side-effects, it's equivalent to an immutable value. If you always call such functions without parentheses, then you're free to change the underlying definition of the function to a value without having to refactor it everywhere it's referenced.
It's worth noting that val definitions are actually modeled as 0-arity methods in scala (unless specified with a private final beforehand).
Scala does something similar with its treatment of arity-1 functions defined on classes. You are allowed to omit the dot and the parenthesis. For example:
case class Foo(value: String) {
def prepend(value2: String) = Foo(value2 + value)
}
Foo("one").prepend("two")
// is the same as...
Foo("one") prepend "two"
This is because Scala models all operators as arity-1 functions. You can rewrite 1 + 2 as 1.+(2) and have it mean the same thing. Representing operators as fully-fledged functions has some nice qualities. You can expect to pass an operator anywhere that you could pass a function, and the definition of the operator is actually defined for class instances (as opposed to a language like C#, where the infix operators are actually static methods that use special syntactic sugar to let them be represented as infix).