Related
Given Scala 2.12.6:
val list = List(1)
val x = 2
This works:
list.map ( y => x + y )
returning List[Int] = List(3)
and this works:
list.map ( (y: Int) => x + y )
returning the same value.
Same for this:
list.map { (y: Int) => x + y }
And same for this:
list.map { y: Int => x + y }
Yet this fails:
list.map ( y: Int => x + y )
producing the error:
error: not found: type +
list.map ( y: Int => x + y )
^
Why is Scala thinking the + is meant to indicate a type, and where is this difference between using parenthesis and curly braces documented and explained?
The Section 6.23 about anonymous functions says:
In the case of a single untyped formal parameter, (x) => e can be abbreviated to x => e. If an anonymous function (x: T) => e with a single typed parameter appears as the result expression of a block, it can be abbreviated to x: T => e.
Thus, in a block { ... }, the function literal (y: Int) => x + y can be abbreviated to just y: Int => x + y.
Without the block, the entire Int => x + y-part is treated as type ascription, so the error message actually makes sense. For example, here is a context in which the offending expression becomes valid:
type y = Unit
type x = Unit
type +[A, B] = Int
val y = (i: Int) => 42 + i
val list = List(1)
println(
list.map ( y: Int => x + y )
) // happily prints `List(43)`.
This is because there are two ys in two separate scopes (one value, one type alias), so that (y: Int => x + y) becomes (y: Int => +[x, y]), and then (y: Int => Int), which is just a type ascription enforcing that value y is indeed of function type Int => Int (which it is, so everything compiles and runs). Here is another similar example.
My suggestion: stick to the slightly more verbose (foo: Foo) => { ... } notation, it will cause fewer surprises for everyone who tries to read and to modify the code. Otherwise there is some risk that
argument types in bindings collide with type ascriptions
=> of the anonymous lambda collides with function type =>
arithmetic operation + collides with binary infix type constructor +[_,_]
values x, y collide with undefined types x, y.
The fact that same syntax can denote both types and expressions can be somewhat of a double-edged sword.
What is the formal difference between passing arguments to functions in parentheses () and in braces {}?
The feeling I got from the Programming in Scala book is that Scala's pretty flexible and I should use the one I like best, but I find that some cases compile while others don't.
For instance (just meant as an example; I would appreciate any response that discusses the general case, not this particular example only):
val tupleList = List[(String, String)]()
val filtered = tupleList.takeWhile( case (s1, s2) => s1 == s2 )
=> error: illegal start of simple expression
val filtered = tupleList.takeWhile{ case (s1, s2) => s1 == s2 }
=> fine.
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) }
There are a couple of different rules and inferences going on here: first of all, Scala infers the braces when a parameter is a function, e.g. in list.map(_ * 2) the braces are inferred, it's just a shorter form of list.map({_ * 2}). Secondly, Scala allows you to skip the parentheses on the last parameter list, if that parameter list has one parameter and it is a function, so list.foldLeft(0)(_ + _) can be written as list.foldLeft(0) { _ + _ } (or list.foldLeft(0)({_ + _}) if you want to be extra explicit).
However, if you add case you get, as others have mentioned, a partial function instead of a function, and Scala will not infer the braces for partial functions, so list.map(case x => x * 2) won't work, but both list.map({case x => 2 * 2}) and list.map { case x => x * 2 } will.
There is an effort from the community to standardize the usage of braces and parentheses, see Scala Style Guide (page 21): http://www.codecommit.com/scala-style-guide.pdf
The recommended syntax for higher order methods calls is to always use braces, and to skip the dot:
val filtered = tupleList takeWhile { case (s1, s2) => s1 == s2 }
For "normal" metod calls you should use the dot and parentheses.
val result = myInstance.foo(5, "Hello")
I don't think there is anything particular or complex about curly braces in Scala. To master the seeming-complex usage of them in Scala, just keep a couple of simple things in mind:
curly braces form a block of code, which evaluates to the last line of code (almost all languages do this)
a function if desired can be generated with the block of code (follows rule 1)
curly braces can be omitted for one-line code except for a case clause (Scala choice)
parentheses can be omitted in function call with code block as a parameter (Scala choice)
Let's explain a couple of examples per the above three rules:
val tupleList = List[(String, String)]()
// doesn't compile, violates case clause requirement
val filtered = tupleList.takeWhile( case (s1, s2) => s1 == s2 )
// block of code as a partial function and parentheses omission,
// i.e. tupleList.takeWhile({ case (s1, s2) => s1 == s2 })
val filtered = tupleList.takeWhile{ case (s1, s2) => s1 == s2 }
// curly braces omission, i.e. List(1, 2, 3).reduceLeft({_+_})
List(1, 2, 3).reduceLeft(_+_)
// parentheses omission, i.e. List(1, 2, 3).reduceLeft({_+_})
List(1, 2, 3).reduceLeft{_+_}
// not both though it compiles, because meaning totally changes due to precedence
List(1, 2, 3).reduceLeft _+_ // res1: String => String = <function1>
// curly braces omission, i.e. List(1, 2, 3).foldLeft(0)({_ + _})
List(1, 2, 3).foldLeft(0)(_ + _)
// parentheses omission, i.e. List(1, 2, 3).foldLeft(0)({_ + _})
List(1, 2, 3).foldLeft(0){_ + _}
// block of code and parentheses omission
List(1, 2, 3).foldLeft {0} {_ + _}
// not both though it compiles, because meaning totally changes due to precedence
List(1, 2, 3).foldLeft(0) _ + _
// error: ';' expected but integer literal found.
List(1, 2, 3).foldLeft 0 (_ + _)
def foo(f: Int => Unit) = { println("Entering foo"); f(4) }
// block of code that just evaluates to a value of a function, and parentheses omission
// i.e. foo({ println("Hey"); x => println(x) })
foo { println("Hey"); x => println(x) }
// parentheses omission, i.e. f({x})
def f(x: Int): Int = f {x}
// error: missing arguments for method f
def f(x: Int): Int = f x
I think it is worth explaining their usage in function calls and why various things happen. As someone already said curly braces define a block of code, which is also an expression so can be put where expression is expected and it will be evaluated. When evaluated, its statements are executed and last's statement value is the result of whole block evaluation (somewhat like in Ruby).
Having that we can do things like:
2 + { 3 } // res: Int = 5
val x = { 4 } // res: x: Int = 4
List({1},{2},{3}) // res: List[Int] = List(1,2,3)
Last example is just a function call with three parameters, of which each is evaluated first.
Now to see how it works with function calls let's define simple function that take another function as a parameter.
def foo(f: Int => Unit) = { println("Entering foo"); f(4) }
To call it, we need to pass function that takes one param of type Int, so we can use function literal and pass it to foo:
foo( x => println(x) )
Now as said before we can use block of code in place of an expression so let's use it
foo({ x => println(x) })
What happens here is that code inside {} is evaluated, and the function value is returned as a value of the block evaluation, this value is then passed to foo. This is semantically the same as previous call.
But we can add something more:
foo({ println("Hey"); x => println(x) })
Now our code block contains two statements, and because it is evaluated before foo is executed, what happens is that first "Hey" is printed, then our function is passed to foo, "Entering foo" is printed and lastly "4" is printed.
This looks a bit ugly though and Scala lets us to skip the parenthesis in this case, so we can write:
foo { println("Hey"); x => println(x) }
or
foo { x => println(x) }
That looks much nicer and is equivalent to the former ones. Here still block of code is evaluated first and the result of evaluation (which is x => println(x)) is passed as an argument to foo.
Because you are using case, you are defining a partial function and partial functions require curly braces.
Increased compile checking with parens
The authors of Spray, recommend that round parens 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 gave it two or more, it will complain. Now this isn't the case with curly braces, if for example, you forget an operator somewhere your code will compile, 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: http://docs.scala-lang.org/style/declarations.html "... 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. E.g. List(1, 2, 3).reduceLeft{_ + _} becomes:
List(1, 2, 3).reduceLeft {
_ + _
}
Parenthesis in an ideal coding style is basically used for single line code.
But if the particular piece of code is multiline then using braces is a better way.
With braces, you got semicolon induced for you and parentheses not. Consider takeWhile function, since it expects partial function, only {case xxx => ??? } is valid definition instead of parentheses around case expression.
I want to iterate over a list of values using a beautiful one-liner in Scala.
For example, this one works well:
scala> val x = List(1,2,3,4)
x: List[Int] = List(1, 2, 3, 4)
scala> x foreach println
1
2
3
4
But if I use the placeholder _, it gives me an error:
scala> x foreach println(_ + 1)
<console>:6: error: missing parameter type for expanded function ((x$1) =>x$1.$plus(1))
x foreach println(_ + 1)
^
Why is that? Can't compiler infer type here?
This:
x foreach println(_ + 1)
is equivalent to this:
x.foreach(println(x$1 => x$1 + 1))
There's no indication as to what might be the type of x$1, and, to be honest, it doesn't make any sense to print a function.
You obviously (to me) meant to print x$0 + 1, where x$0 would the the parameter passed by foreach, instead. But, let's consider this... foreach takes, as a parameter, a Function1[T, Unit], where T is the type parameter of the list. What you are passing to foreach instead is println(_ + 1), which is an expression that returns Unit.
If you wrote, instead x foreach println, you'd be passing a completely different thing. You'd be passing the function(*) println, which takes Any and returns Unit, fitting, therefore, the requirements of foreach.
This gets slightly confused because of the rules of expansion of _. It expands to the innermost expression delimiter (parenthesis or curly braces), except if they are in place of a parameter, in which case it means a different thing: partial function application.
To explain this better, look at these examples:
def f(a: Int, b: Int, c: Int) = a + b + c
val g: Int => Int = f(_, 2, 3) // Partial function application
g(1)
Here, we applies the second and third arguments to f, and returned a function requiring just the remaining argument. Note that it only worked as is because I indicated the type of g, otherwise I'd have to indicate the type of the argument I was not applying. Let's continue:
val h: Int => Int = _ + 1 // Anonymous function, expands to (x$1: Int => x$1 + 1)
val i: Int => Int = (_ + 1) // Same thing, because the parenthesis are dropped here
val j: Int => Int = 1 + (_ + 1) // doesn't work, because it expands to 1 + (x$1 => x$1 + 1), so it misses the type of `x$1`
val k: Int => Int = 1 + ((_: Int) + 1) // doesn't work, because it expands to 1 + (x$1: Int => x$1 + 1), so you are adding a function to an `Int`, but this operation doesn't exist
Let discuss k in more detail, because this is a very important point. Recall that g is a function Int => Int, right? So, if I were to type 1 + g, would that make any sense? That's what was done in k.
What confuses people is that what they really wanted was:
val j: Int => Int = x$1 => 1 + (x$1 + 1)
In other words, they want the x$1 replacing _ to jump to outside the parenthesis, and to the proper place. The problem here is that, while it may seem obvious to them what the proper place is, it is not obvious to the compiler. Consider this example, for instance:
def findKeywords(keywords: List[String], sentence: List[String]) = sentence.filter(keywords contains _.map(_.toLowerCase))
Now, if we were to expand this to outside the parenthesis, we would get this:
def findKeywords(keywords: List[String], sentence: List[String]) = (x$1, x$2) => sentence.filter(keywords contains x$1.map(x$2.toLowerCase))
Which is definitely not what we want. In fact, if the _ did not get bounded by the innermost expression delimiter, one could never use _ with nested map, flatMap, filter and foreach.
Now, back to the confusion between anonymous function and partial application, look here:
List(1,2,3,4) foreach println(_) // doesn't work
List(1,2,3,4) foreach (println(_)) // works
List(1,2,3,4) foreach (println(_ + 1)) // doesn't work
The first line doesn't work because of how operation notation works. Scala just sees that println returns Unit, which is not what foreachexpects.
The second line works because the parenthesis let Scala evaluate println(_) as a whole. It is a partial function application, so it returns Any => Unit, which is acceptable.
The third line doesn't work because _ + 1 is anonymous function, which you are passing as a parameter to println. You are not making println part of an anonymous function, which is what you wanted.
Finally, what few people expect:
List(1,2,3,4) foreach (Console println _ + 1)
This works. Why it does is left as an exercise to the reader. :-)
(*) Actually, println is a method. When you write x foreach println, you are not passing a method, because methods can't be passed. Instead, Scala creates a closure and passes it. It expands like this:
x.foreach(new Function1[Any,Unit] { def apply(x$1: Any): Unit = Console.println(x$1) })
The underscore is a bit tricky. According to the spec, the phrase:
_ + 1
is equivalent to
x => x + 1
Trying
x foreach println (y => y + 1)
yields:
<console>:6: error: missing parameter type
x foreach println (y => y + 1)
If you add some types in:
x foreach( println((y:Int) => y + 1))
<console>:6: error: type mismatch;
found : Unit
required: (Int) => Unit
x foreach( println((y:Int) => y + 1))
The problem is that you are passing an anonymous function to println and it's not able to deal with it. What you really want to do (if you are trying to print the successor to each item in the list) is:
x map (_+1) foreach println
scala> for(x <- List(1,2,3,4)) println(x + 1)
2
3
4
5
There is a strange limitation in Scala for the nesting depth of expressions with underscore. It's well seen on the following example:
scala> List(1) map(1+_)
res3: List[Int] = List(2)
scala> Some(1) map (1+(1+_))
<console>:5: error: missing parameter type for expanded function ((x$1) => 1.+(x$1))
Some(1) map (1+(1+_))
^
Looks like a bug for me.
Welcome to Scala version 2.8.0.Beta1-prerelease (Java HotSpot(TM) Client VM, Java 1.6.0_17).
Type in expressions to have them evaluated.
Type :help for more information.
scala> val l1 = List(1, 2, 3)
l1: List[Int] = List(1, 2, 3)
scala>
scala> l1.foreach(println(_))
1
2
3
As far as I know, the infix operator usage in Scala should be equivalent to the invocation of a method. So:
scala> "a" + 3.toString
res0: java.lang.String = a3
Is the same as:
scala> "a".+(3.toString)
res1: java.lang.String = a3
I came across an occasion where this is not happening, when there is a placeholder. I was doing something more complex, but it can be distilled to:
scala> def x(f:(Int)=>String) = f(3)
x: (f: Int => String)String
scala> x("a" + _.toString)
res3: String = a3
So far so good. But...
scala> x("a".+(_.toString))
<console>:9: error: missing parameter type for expanded function ((x$1) => x$1.toString)
x("a".+(_.toString))
What's the difference here? What am I missing?
Jordi
The _ placeholder can only appear at the topmost Expr in its function. That means
(_.toString)
is itself a function, and "a" + some function of unknown type doesn't make much sense to the compiler.
Your assessment of infix notation is correct, but your understanding of placeholder parameters is flawed.
When you use underscore as a placeholder parameter, you are creating a function. The question is what are the boundaries of that function: where does it start, where does it end? For example, consider this expression:
_ + _ + _
How should it be translated? Here are some alternatives:
(x, y, z) => { x + y + z }
(x, y) => { (z) => { x + y } + z }
(x) => { x + { (y, z) => y + z } }
Well, Scala rule is that the scope is the innermost parenthesis-delimited expression, or the whole expression otherwise. So, in practice, you wrote two different things:
x("a" + _.toString) // is the same thing as
x((y) => "a" + y.toString)
x("a".+(_.toString)) // is the same thing as
x("a".+((y) => y.toString))
I want to iterate over a list of values using a beautiful one-liner in Scala.
For example, this one works well:
scala> val x = List(1,2,3,4)
x: List[Int] = List(1, 2, 3, 4)
scala> x foreach println
1
2
3
4
But if I use the placeholder _, it gives me an error:
scala> x foreach println(_ + 1)
<console>:6: error: missing parameter type for expanded function ((x$1) =>x$1.$plus(1))
x foreach println(_ + 1)
^
Why is that? Can't compiler infer type here?
This:
x foreach println(_ + 1)
is equivalent to this:
x.foreach(println(x$1 => x$1 + 1))
There's no indication as to what might be the type of x$1, and, to be honest, it doesn't make any sense to print a function.
You obviously (to me) meant to print x$0 + 1, where x$0 would the the parameter passed by foreach, instead. But, let's consider this... foreach takes, as a parameter, a Function1[T, Unit], where T is the type parameter of the list. What you are passing to foreach instead is println(_ + 1), which is an expression that returns Unit.
If you wrote, instead x foreach println, you'd be passing a completely different thing. You'd be passing the function(*) println, which takes Any and returns Unit, fitting, therefore, the requirements of foreach.
This gets slightly confused because of the rules of expansion of _. It expands to the innermost expression delimiter (parenthesis or curly braces), except if they are in place of a parameter, in which case it means a different thing: partial function application.
To explain this better, look at these examples:
def f(a: Int, b: Int, c: Int) = a + b + c
val g: Int => Int = f(_, 2, 3) // Partial function application
g(1)
Here, we applies the second and third arguments to f, and returned a function requiring just the remaining argument. Note that it only worked as is because I indicated the type of g, otherwise I'd have to indicate the type of the argument I was not applying. Let's continue:
val h: Int => Int = _ + 1 // Anonymous function, expands to (x$1: Int => x$1 + 1)
val i: Int => Int = (_ + 1) // Same thing, because the parenthesis are dropped here
val j: Int => Int = 1 + (_ + 1) // doesn't work, because it expands to 1 + (x$1 => x$1 + 1), so it misses the type of `x$1`
val k: Int => Int = 1 + ((_: Int) + 1) // doesn't work, because it expands to 1 + (x$1: Int => x$1 + 1), so you are adding a function to an `Int`, but this operation doesn't exist
Let discuss k in more detail, because this is a very important point. Recall that g is a function Int => Int, right? So, if I were to type 1 + g, would that make any sense? That's what was done in k.
What confuses people is that what they really wanted was:
val j: Int => Int = x$1 => 1 + (x$1 + 1)
In other words, they want the x$1 replacing _ to jump to outside the parenthesis, and to the proper place. The problem here is that, while it may seem obvious to them what the proper place is, it is not obvious to the compiler. Consider this example, for instance:
def findKeywords(keywords: List[String], sentence: List[String]) = sentence.filter(keywords contains _.map(_.toLowerCase))
Now, if we were to expand this to outside the parenthesis, we would get this:
def findKeywords(keywords: List[String], sentence: List[String]) = (x$1, x$2) => sentence.filter(keywords contains x$1.map(x$2.toLowerCase))
Which is definitely not what we want. In fact, if the _ did not get bounded by the innermost expression delimiter, one could never use _ with nested map, flatMap, filter and foreach.
Now, back to the confusion between anonymous function and partial application, look here:
List(1,2,3,4) foreach println(_) // doesn't work
List(1,2,3,4) foreach (println(_)) // works
List(1,2,3,4) foreach (println(_ + 1)) // doesn't work
The first line doesn't work because of how operation notation works. Scala just sees that println returns Unit, which is not what foreachexpects.
The second line works because the parenthesis let Scala evaluate println(_) as a whole. It is a partial function application, so it returns Any => Unit, which is acceptable.
The third line doesn't work because _ + 1 is anonymous function, which you are passing as a parameter to println. You are not making println part of an anonymous function, which is what you wanted.
Finally, what few people expect:
List(1,2,3,4) foreach (Console println _ + 1)
This works. Why it does is left as an exercise to the reader. :-)
(*) Actually, println is a method. When you write x foreach println, you are not passing a method, because methods can't be passed. Instead, Scala creates a closure and passes it. It expands like this:
x.foreach(new Function1[Any,Unit] { def apply(x$1: Any): Unit = Console.println(x$1) })
The underscore is a bit tricky. According to the spec, the phrase:
_ + 1
is equivalent to
x => x + 1
Trying
x foreach println (y => y + 1)
yields:
<console>:6: error: missing parameter type
x foreach println (y => y + 1)
If you add some types in:
x foreach( println((y:Int) => y + 1))
<console>:6: error: type mismatch;
found : Unit
required: (Int) => Unit
x foreach( println((y:Int) => y + 1))
The problem is that you are passing an anonymous function to println and it's not able to deal with it. What you really want to do (if you are trying to print the successor to each item in the list) is:
x map (_+1) foreach println
scala> for(x <- List(1,2,3,4)) println(x + 1)
2
3
4
5
There is a strange limitation in Scala for the nesting depth of expressions with underscore. It's well seen on the following example:
scala> List(1) map(1+_)
res3: List[Int] = List(2)
scala> Some(1) map (1+(1+_))
<console>:5: error: missing parameter type for expanded function ((x$1) => 1.+(x$1))
Some(1) map (1+(1+_))
^
Looks like a bug for me.
Welcome to Scala version 2.8.0.Beta1-prerelease (Java HotSpot(TM) Client VM, Java 1.6.0_17).
Type in expressions to have them evaluated.
Type :help for more information.
scala> val l1 = List(1, 2, 3)
l1: List[Int] = List(1, 2, 3)
scala>
scala> l1.foreach(println(_))
1
2
3