I'd like to call a function returned by a function with an implicit parameter, simply and elegantly. This doesn't work:
def resolveA(implicit a: A): String => String = { prefix =>
s"$prefix a=$a"
}
case class A(n: Int)
implicit val a = A(1)
println(resolveA("-->")) // won't compile
I've figured out what's going on: Scala sees the ("-->") and thinks it's an attempt to explicitly fill in the implicit parameter list. I want to pass that as the prefix argument, but Scala sees it as the a argument.
I've tried some alternatives, like putting an empty parameter list () before the implicit one, but so far I've always been stopped by the fact that Scala thinks the argument to the returned function is an attempt to fill in the implicit parameter list of resolveA.
What's a nice way to do what I'm trying to do here, even if it's not as nice as the syntax I tried above?
Another option would be to use the apply method of the String => String function returned by resolveA. This way the compiler won't confuse the parameter lists, and is a little shorter than writing implicltly[A].
scala> resolveA[A].apply("-->")
res3: String = --> a=A(1)
Related
I have a method that with the implicit parameter. i get a error when i convert it to function in 2 case :
1:
def action(implicit i:Int) = i + " in action"
val f = action _
then i get a StackOverflowError.
2：
def action(implicit i:Int) = i + " in action"
val f = action(_)
then i get a error: missing parameter type
I must write like this :
val f = (i:Int) => action(i)
that's ok. And if the parameter of 'action' is not the implicit , all case are right. So how to explain , and what i miss ?
If you specify a parameter to a function to be implicit, you are inviting the compiler to supply the value of that parameter for you. So how does the compiler find those values? It looks for values of the same type (Int in your case) that have been declared as implicit values in a variety of scopes.
(For simplicity, I'll just use a local scope in this example, but you might want to read up on this topic. Programming in Scala, 3rd Ed is a good first step.)
Note that the names of the implicit values are ignored and have no bearing on proceedings, the compiler only looks at the types of implicit values. If multiple implicit values with the required type are found in the same scope, then the compiler will complain about ambiguous implicit values.
For example, the following provides a function with an implicit parameter and a default value for that parameter within the current scope:
def greetPerson(name: String)(implicit greeting: String) = s"$greeting $name!"
implicit val defaultGreeting = "Hello" // Implicit value to be used for greeting argument.
val y = greetPerson("Bob") // Equivalent to greetPerson("Bob")(defaultGreeting).
val z = greetPerson("Fred")("Hi")
Note that y is just a String value of "Hello Bob!", and z is a string with the value "Hi Fred!"; neither of them are functions.
Also note that greetPerson is a curried function. This is because implicit parameters cannot be mixed with regular, non-implicit parameters in the same parameter list.
In general, it's bad practice to use common types (Int, Boolean, String, etc.) as values for implicit parameters. In a big program, there might be a lot of different implicit values in your scope, and you might pick up an unexpected value. For that reason, it's standard practice to wrap such values in a case class instead.
If you're trying to create a value that supplies some of the arguments of another function (that is, a partially applied function), then that would look something like this:
def greetPerson(greeting: String, name: String) = s"$greeting $name!"
val sayHello = greetPerson("Hello", _: String)
val y = sayHello("Bob") // "Hello Bob!"
val sayHi = greetPerson("Hi", _: String)
val z = sayHi("Fred") // "Hi Fred!"
In both cases, we're creating partially applied functions (sayHi and sayHello) that call greetPerson with the greeting parameter specified, but which allow us to specify the name parameter. Both sayHello and sayHi are still only values, but their values are partially applied functions rather than constants.
Depending upon your circumstances, I think the latter case may suit you better...
I would also read up on how the underscore character (_) is used in Scala. In a partially applied function declaration, it corresponds to the arguments that will be provided later. But it has a lot of other uses too. I think there's no alternative to reading up on Scala and learning how and when to use them.
I've written a simple code in Scala with implicit conversion of Function1 to some case class.
object MyApp extends App{
case class FunctionContainer(val function:AnyRef)
implicit def cast(function1: Int => String):FunctionContainer = new FunctionContainer(function1)
def someFunction(i:Int):String = "someString"
def abc(f : FunctionContainer):String = "abc"
println(abc(someFunction))
}
But it doesn't work. Compiler doesn't want to pass someFunction as an argument to abc. I can guess its reasons but don't know exactly why it doesn't work.
When you use a method name as you have, the compiler has to pick how to convert the method type to a value. If the expected type is a function, then it eta-expands; otherwise it supplies empty parens to invoke the method. That is described here in the spec.
But it wasn't always that way. Ten years ago, you would have got your function value just by using the method name.
The new online spec omits the "Change Log" appendix, so for the record, here is the moment when someone got frustrated with parens and introduced the current rules. (See Scala Reference 2.9, page 181.)
This has not eliminated all irksome anomalies.
Conversions
The rules for implicit conversions of methods to functions (§6.26) have been tightened. Previously, a parameterized method used as a value was always implicitly converted to a function. This could lead to unexpected results when method arguments were forgotten. Consider for instance the statement below:
show(x.toString)
where show is defined as follows:
def show(x: String) = Console.println(x)
Most likely, the programmer forgot to supply an empty argument list () to toString. The previous Scala version would treat this code as a partially applied method, and expand it to:
show(() => x.toString())
As a result, the address of a closure would be printed instead of the value of s. Scala version 2.0 will apply a conversion from partially applied method to function value only if the expected type of the expression is indeed a function type. For instance, the conversion would not be applied in the code above because the expected type of show’s parameter is String, not a function type. The new convention disallows some previously legal code. Example:
def sum(f: int => double)(a: int, b: int): double =
if (a > b) 0 else f(a) + sum(f)(a + 1, b)
val sumInts = sum(x => x) // error: missing arguments
The partial application of sum in the last line of the code above will not be converted to a function type. Instead, the compiler will produce an error message which states that arguments for method sum are missing. The problem can be fixed by providing an expected type for the partial application, for instance by annotating the definition of sumInts with its type:
val sumInts: (int, int) => double = sum(x => x) // OK
On the other hand, Scala version 2.0 now automatically applies methods with empty parameter lists to () argument lists when necessary. For instance, the show expression above will now be expanded to
show(x.toString())
Your someFunction appears as a method here.
You could try either
object MyApp extends App{
case class FunctionContainer(val function:AnyRef)
implicit def cast(function1: Int => String):FunctionContainer = new FunctionContainer(function1)
val someFunction = (i:Int) => "someString"
def abc(f : FunctionContainer):String = "abc"
println(abc(someFunction))
}
or
object MyApp extends App{
case class FunctionContainer(val function:AnyRef)
implicit def cast(function1: Int => String):FunctionContainer = new FunctionContainer(function1)
def someFunction(i:Int): String = "someString"
def abc(f : FunctionContainer):String = "abc"
println(abc(someFunction(_: Int)))
}
By the way: implicitly casting such common functions to something else can quickly lead to problems. Are you absolutely sure that you need this? Wouldn't it be easier to overload abc?
You should use eta-expansion
println(abc(someFunction _))
Excuse the long set-up. This question relates to, but is not answered by, Scala: ambiguous reference to overloaded definition - best disambiguation? .
I'm pretty new to Scala, and one thing that's throwing me off is that Scala both:
Has first-class functions
Calls functions when using object-dot notation without any parenthetical argument lists (as if the function were a property)
These two language features are confusing me. Look at the below code:
class MyClass {
def something(in: String): String = {
in + "_X"
}
def something: String => String = {
case _ => "Fixed"
}
}
val my = new MyClass()
println(List("foo", "bar").map(my.something))
I would expect this to print List("foo_X", "bar_X") by calling the something prototype that matches the map's required String => ? argument. Instead, the output is List("Fixed", "Fixed") - Scala 2.11 is invoking the no-argument something() and then passing its return value to the map.
If we comment out the second no-argument prototype of something, the output changes to be the expected result, demonstrating that the other prototype is valid in context.
Adding an empty argument list to the second prototype (making it def something()) also changes the behavior.
Changing the my.something to my.something(_) wakes Scala up to the ambiguity it was silently ignoring before:
error: ambiguous reference to overloaded definition,
both method something in class MyClass of type => String => String
and method something in class MyClass of type (in: String)String
match argument types (String)
println(List("foo", "bar").map(my.something(_)))
Even using the supposedly-for-this-purpose magic trailing underscore doesn't work:
val myFun: (String) => String = my.something _
This results in:
error: type mismatch;
found : () => String => String
required: String => String
val myFun: (String) => String = my.something _
My questions:
If I have MyClass exactly as written (no changes to the prototypes, especially not adding an empty parameter list to one of the prototypes), how do I tell Scala, unambiguously, that I want the first one-argument version of something to pass as an argument to another call?
Since there are clearly two satisfying arguments that could be passed to map, why did the Scala compiler not report the ambiguity as an error?
Is there a way to disable Scala's behavior of (sometimes, not always) treating foo.bar as equivalent to foo.bar()?
I have filed a bug on the Scala issue tracker and the consensus seems to be that this behaviour is a bug. The compiler should have thrown an error about the ambiguous reference to "my.something".
I was looking at the FoldLeft and FoldRight methods and the operator version of the method was extremely peculiar which was something like this (0 /: List.range(1,10))(+).
For right associative functions with two parameter lists one would expect the syntax to be something like this((param1)(param2) op HostClass).
But here in this case it is of the syntax (param1 op HostClass)(param2). This causes ambiguity with another case where a right associative function returns another function that takes a single parameter.
Because of this ambiguity the class compiles but fails when the function call is made as shown below.
class Test() {
val func1:(String => String) = { (in) => in * 2 }
def `test:`(x:String) = { println(x); func1 }
def `test:`(x:String)(y:String) = { x+" "+y }
}
val test = new Test
(("Foo") `test:` test)("hello")
<console>:10: error: ambiguous reference to overloaded definition,
both method test: in class Test of type (x: String)(y: String)String
and method test: in class Test of type (x: String)String => String
match argument types (String)
(("Foo") `test:` test)("hello")
so my questions are
Is this an expected behaviour or is it a bug?
Why the two parameter list right associative function call has been designed the way it is, instead of what I think to be more intuitive syntax of ((param1)(param2) op HostClass)?
Is there a workaround to call either of the overloaded test: function without ambiguity.
The Scala's Type System considers only the first parameter list of the function for type inference. Hence to uniquely identify one of the overloaded method in a class or object the first parameter list of the method has to be distinct for each of the overloaded definition. This can be demonstrated by the following example.
object Test {
def test(x:String)(y:Int) = { x+" "+y.toString() }
def test(x:String)(y:String) = { x+" "+y }
}
Test.test("Hello")(1)
<console>:9: error: ambiguous reference to overloaded definition,
both method test in object Test of type (x: String)(y: String)String
and method test in object Test of type (x: String)(y: Int)String
match argument types (String)
Test.test("Hello")(1)
Does it really fail at runtime? When I tested it, the class compiles, but the call of the method test: does not.
I think that the problem is not with the operator syntax, but with the fact that you have two overloaded functions, one with just one and the other with two parameter lists.
You will get the same error with the dot-notation:
test.`test:`("Foo")("hello")
If you rename the one-param list function, the ambiguity will be gone and
(("Foo") `test:` test)("hello")
will compile.
Working in Scala-IDE, I have a Java library, in which one of the methods receives java.lang.Object. And I want to map a list of Int values to it. The only solution that works is:
val listOfInts = groupOfObjects.map(_.getNeededInt)
for(int <- listOfInts) libraryObject.libraryMethod(int)
while the following one:
groupOfObjects.map(_.getNeededInt).map(libraryMethod(_)
and even
val listOfInts = groupOfObjects.map(_.getNeededInt)
val result = listOfInts.map(libraryObject.libraryMethod(_))
say
type mismatch; found : Int required: java.lang.Object Note: an
implicit exists from scala.Int => java.lang.Integer, but methods
inherited from Object are rendered ambiguous. This is to avoid a
blanket implicit which would convert any scala.Int to any AnyRef. You
may wish to use a type ascription: x: java.lang.Integer.
and something like
val result = listOfInts.map(libraryObject.libraryMethod(x => x.toInt))
or
val result = listOfInts.map(libraryObject.libraryMethod(_.toInt))
does not work also.
1) Why is it happening? As far as I know, the for and map routines do not differ that much!
2) Also: what means You may wish to use a type ascription: x: java.lang.Integer? How would I do that? I tried designating the type explicitly, like x: Int => x.toInt, but that is too erroneus. So what is the "type ascription"?
UPDATE:
The solution proposed by T.Grottker, adds to it. The error that I am getting with it is this:
missing parameter type for expanded function ((x$3) => x$3.asInstanceOf[java.lang.Object])
missing parameter type for expanded function ((x$3) => x$3.asInstanceOf{#null#}[java.lang.Object]{#null#}) {#null#}
and I'm like, OMG, it just grows! Who can explain what all these <null> things mean here? I just want to know the truth. (NOTE: I had to replace <> brakets with # because the SO engine cut out the whole thing then, so use your imagination to replace them back).
The type mismatch tells you exactly the problem: you can convert to java.lang.Integer but not to java.lang.Object. So tell it you want to ask for an Integer somewhere along the way. For example:
groupOfObjects.map(_.getNeededInt: java.lang.Integer).map(libraryObject.libraryMethod(_))
(The notation value: Type--when used outside of the declaration of a val or var or parameter method--means to view value as that type, if possible; value either needs to be a subclass of Type, or there needs to be an implicit conversion that can convert value into something of the appropriate type.)