I tried once to write about this, but I gave up in the end, as the rules are somewhat diffuse. Basically, you’ll have to get the hang of it.
Perhaps it is best to concentrate on where curly braces and parentheses can be used interchangeably: when passing parameters to method calls. You may replace curly braces with parentheses if, and only if, the method expects a single parameter. For example:
List(1, 2, 3).reduceLeft{_ + _} // valid, single Function2[Int,Int] parameter
List{1, 2, 3}.reduceLeft(_ + _) // invalid, A* vararg parameter
However, there’s more you need to know to better grasp these rules.
Increased compile checking with parens
The authors of Spray recommend round parens because they give increased compile checking. This is especially important for DSLs like Spray. By using parens you are telling the compiler that it should only be given a single line; therefore if you accidentally give it two or more, it will complain. Now this isn’t the case with curly braces – if for example you forget an operator somewhere, then your code will compile, and you get unexpected results and potentially a very hard bug to find. Below is contrived (since the expressions are pure and will at least give a warning), but makes the point:
method {
1 +
2
3
}
method(
1 +
2
3
)
The first compiles, the second gives error: ')' expected but integer literal found. The author wanted to write 1 + 2 + 3.
One could argue it’s similar for multi-parameter methods with default arguments; it’s impossible to accidentally forget a comma to separate parameters when using parens.
Verbosity
An important often overlooked note about verbosity. Using curly braces inevitably leads to verbose code since the Scala style guide clearly states that closing curly braces must be on their own line:
… the closing brace is on its own line immediately following the last
line of the function.
Many auto-reformatters, like in IntelliJ, will automatically perform this reformatting for you. So try to stick to using round parens when you can.
Infix Notation
When using infix notation, like List(1,2,3) indexOf (2) you can omit parentheses if there is only one parameter and write it as List(1, 2, 3) indexOf 2. This is not the case of dot-notation.
Note also that when you have a single parameter that is a multi-token expression, like x + 2 or a => a % 2 == 0, you have to use parentheses to indicate the boundaries of the expression.
Tuples
Because you can omit parentheses sometimes, sometimes a tuple needs extra parentheses like in ((1, 2)), and sometimes the outer parentheses can be omitted, like in (1, 2). This may cause confusion.
Function/Partial Function literals with case
Scala has a syntax for function and partial function literals. It looks like this:
{
case pattern if guard => statements
case pattern => statements
}
The only other places where you can use case statements are with the match and catch keywords:
object match {
case pattern if guard => statements
case pattern => statements
}
try {
block
} catch {
case pattern if guard => statements
case pattern => statements
} finally {
block
}
You cannot use case statements in any other context. So, if you want to use case, you need curly braces. In case you are wondering what makes the distinction between a function and partial function literal, the answer is: context. If Scala expects a function, a function you get. If it expects a partial function, you get a partial function. If both are expected, it gives an error about ambiguity.
Expressions and Blocks
Parentheses can be used to make subexpressions. Curly braces can be used to make blocks of code (this is not a function literal, so beware of trying to use it like one). A block of code consists of multiple statements, each of which can be an import statement, a declaration or an expression. It goes like this:
{
import stuff._
statement ; // ; optional at the end of the line
statement ; statement // not optional here
var x = 0 // declaration
while (x < 10) { x += 1 } // stuff
(x % 5) + 1 // expression
}
( expression )
So, if you need declarations, multiple statements, an import or anything like that, you need curly braces. And because an expression is a statement, parentheses may appear inside curly braces. But the interesting thing is that blocks of code are also expressions, so you can use them anywhere inside an expression:
( { var x = 0; while (x < 10) { x += 1}; x } % 5) + 1
So, since expressions are statements, and blocks of codes are expressions, everything below is valid:
1 // literal
(1) // expression
{1} // block of code
({1}) // expression with a block of code
{(1)} // block of code with an expression
({(1)}) // you get the drift...
Where they are not interchangeable
Basically, you can’t replace {} with () or vice versa anywhere else. For example:
while (x < 10) { x += 1 }
This is not a method call, so you can’t write it in any other way. Well, you can put curly braces inside the parentheses for the condition, as well as use parentheses inside the curly braces for the block of code:
while ({x < 10}) { (x += 1) }
