Suppose I have this code for extracting the code initialising a variable:
def extractBodyImpl[T: Type](expr: Expr[T])(using Quotes) =
import quotes.reflect._
expr.asTerm.underlyingArgument match
case ident # Ident(_) =>
ident.symbol.tree match
case ValDef(_,_,rhs) => println(rhs)
case DefDef(_,_,_,rhs) => println(rhs)
'{ () }
inline def extractBody[T](inline expr: T) = ${ extractBodyImpl('expr) }
When called on a variable declared in the same scope it works as desired:
#main def hello() =
val x = 1
extractBody(x)
prints Some(Literal(Constant(1))).
However, on a variable from outer scope, it prints None:
val x = 1
#main def hello() =
extractBody(x)
How can I make it work in the second case?
In Scala 3 you just need to switch on
scalacOptions += "-Yretain-trees"
Then
val x = 1
#main def hello() =
extractBody(x)
will print Some(Literal(Constant(1))) too.
In Scala 2 we had to use Traverser technique in order to get RHS of definition
Get an scala.MatchError: f (of class scala.reflect.internal.Trees$Ident) when providing a lambda assigned to a val
Def Macro, pass parameter from a value
Creating a method definition tree from a method symbol and a body
Scala macro how to convert a MethodSymbol to DefDef with parameter default values?
How to get the runtime value of parameter passed to a Scala macro?
Can you implement dsinfo in Scala 3? (Can Scala 3 macros get info about their context?) (Scala 3)
You cannot do it in macro. A function which received argument might have been called from everywhere. How would static analysis would access the information only available in runtime? The only reliable solution would be to force user to expand this extractBody macro right after defining the value and passing the result in some wrapper combining both value and its origin.
Related
In a Scala 3 macro that takes a type parameter T, you can use TypeRepr.of[T] and the new Scala 3 reflection API to explore the companionClass of T, and find the Symbol for an arbitrary method on that companion class (eg companionClass.declarations.find(_.name == "list") to find a list() method).
Given the Symbol for a companion object method, how would you then invoke that method within a quoted code block?
I'm guessing I would need to convert that Symbol to a Expr[T], but I don't know how to do that!
In a Scala 2 macro, the invocation of a listMethod of type c.universe.Symbol in a q"..." quasiquote seems pretty simple - just say $listMethod, and then you can start mapping on the resulting list, eg:
q"""
$listMethod.map(_.toString)
"""
Trying to do a similar thing in a Scala 3 macro gets an error like this:
[error] 27 | ${listMethod}.map(_.toString)
[error] | ^^^^^^^^^^
[error] | Found: (listMethod : x$1.reflect.Symbol)
[error] | Required: quoted.Expr[Any]
What is the correct code to get this working in Scala 3?
You can see more code context in the AvroSerialisableMacro classes (Scala 2 compiles, Scala 3 currently nowhere near!) here: https://github.com/guardian/marley/pull/77/files
First, let's talk how to call a method using symbol name in general.
You might need Select. You can call obtain it in a a few different ways, e.g.:
New(TypeTree.of[YourType]).select(primaryConstructor) // when you want to create something
expression.asTerm.select(method) // when you want to call it on something
Once you selected method you can provide arguments:
select.appliedToArgs(args) // if there is only 1 argument list
select.appliedToArgss(args) // if there is more than one argument list
// (type parameter list is listed in paramSymss
// but shouldn't be used here, so filter it out!)
select.appliedToNone // if this is a method like "def method(): T"
// (single, but empty, parameter list)
select.appliedToArgss(Nil) // is this is a method like "def method: T"
// (with not even empty parameter list)
There are also other methods like appliedToType, appliedToTypeTrees, but if you have a method name as a Symbol and want to use it to call something this should be a good starting point.
And remember that source code of Quotes is your friend, so even when your IDE doesn't give you any suggestions, it can point you towards some solution.
In theory these methods are defined on Term rather than Select (<: Term) but your use case will be most likely picking an expression and calling a method on it with some parameters. So a full example could be e.g.
val expression: Expr[Input]
val method: Symbol
val args: List[Term]
// (input: Input).method(args) : Output
expression // Expr[Input]
.asTerm // Term
.select(method) // Select
.appliedToArgs(args) // Term
.asExpr // Expr[?]
.asExprOf[Output] // Expr[Output]
Obviously, proving that the expression can call method and making sure that types of Terms in args match allowed types of values that you pass to the method, is on you. It is a bit more hassle than it was in Scala 2 since quotes allow you to work with Type[T] and Expr[T] only, so anything that doesn't fall under that category has to be implemented with macros/Tasty ADT until you get to the point that you can return Expr inside ${}.
That said, the example you linked shows that these calls are rather hardcoded, so you don't have to look up Symbols and call them. Your code will most likely do away with:
// T <: ThriftEnum
// Creating companion's Expr can be done with asExprOf called on
// Ref from Dmytro Mitin's answer
def findCompanionOfThisOrParent(): Expr[ThriftEnumObject[T]] = ...
// _Expr_ with the List value you constructed instead of Symbol!
val listOfValues: Expr[List[T]] = '{
${ findCompanionOfThisOrParent() }.list
}
// once you have an Expr you don't have to do any magic
// to call a method on it, Quotes works nice
'{
...
val valueMap = Map(${ listOfValues }.map(x => x ->
org.apache.avro.generic.GenericData.get.createEnum(
com.gu.marley.enumsymbols.SnakesOnACamel.toSnake(x.name), schemaInstance)
): _*)
...
}
Difference between Scala 2 quasiquotes and Scala 3 quotations is that the former must compile during compile time of the main code using macros (i.e. during macro expansion, macro runtime) while the latter must compile earlier, at macro compile time. So Scala 3 quotations '{...}/${...} are more like Scala 2 reify{...}/.splice than Scala 2 quasiquotes q"..."/${...}.
`tq` equivalent in Scala 3 macros
You have to re-create AST. Let's see what shape AST should have:
object B:
def fff(): Unit = ()
import scala.quoted.*
inline def foo(): Unit = ${fooImpl}
def fooImpl(using Quotes): Expr[Unit] =
import quotes.reflect.*
println('{B.fff()}.asTerm.show(using Printer.TreeStructure))
'{()}
foo() // ... Apply(Select(Ident("B"), "fff"), Nil)
So in order to re-create AST try to use Apply(...) and Select.unique(..., "list"):
import scala.quoted.*
inline def foo[T](): Unit = ${fooImpl[T]}
def fooImpl[T: Type](using Quotes): Expr[Unit] =
import quotes.reflect.*
val sym = TypeRepr.of[T].typeSymbol
'{
println("aaa")
${
Apply(
Select.unique(
Ref(sym.companionModule),
"list"
),
Nil
).asExprOf[Unit]
}
}
Testing (in a different file):
class A
object A {
def list(): Unit = println("list")
}
foo[A]()
//scalac: {
// scala.Predef.println("aaa")
// A.list()
//}
// prints at runtime:
// aaa
// list
Using method symbol rather than its name and using convenience methods rather than AST nodes directly, you can rewrite fooImpl as
def fooImpl[T: Type](using Quotes): Expr[Unit] =
import quotes.reflect.*
val sym = TypeRepr.of[T].typeSymbol
val listMethod = sym.companionClass.declarations.find(_.name == "list").get
'{
println("aaa")
${
Ref(sym.companionModule)
.select(listMethod)
.appliedToArgs(Nil)
.asExprOf[Unit]
}
}
This is just an example how to create an AST. You should use your actual return type of def list() instead of Unit in .asExprOf[Unit].
How to get the list of default fields values for typed case class?
scala 3 macro how to implement generic trait
I'm wondering if it is possible to write a macro in Scala 3 that take a set of strings and turn it into an enum type with with those strings as cases?
For example, I would like to write a class with an internal type generated from the input element:
import scala.quoted.*
class Example(myEnumElements:Seq[String]) {
inline def buildEnum(inline elts:Seq[String]): Unit = ${ buildEnumType('elts) }
def buildEnumType(e: Expr[Seq[String]])(using Quotes, Type[Seq]): Expr[Unit] = '{
enum MyEnum:
???
}
}
...
// Possibly in another file?
val example = Example(Seq("A","B","C"))
def someConvenienceFunction(e:example.MyEnum) = e match
case A => "apple"
case B => "banana"
case C => "cranberry"
...
// Possibly in another file?
someConvenienceFunction(example.A) // "apple"
someConvenienceFunction(example.D) // compile error
Scala 3 macros are currently only def macros. They are not for generating classes, enums etc. Even if you define an enum inside buildEnumType it will be visible only inside the block {...} that buildEnum call expands into.
Try to use code generation instead.
How to generate a class in Dotty with macro?
https://users.scala-lang.org/t/macro-annotations-replacement-in-scala-3/7374
How to create variables with macros in Scala (Scala 2)
Resolving variables in scope modified by Scala macro (Scala 2)
How possible that the first is correct Scala code but the second won't even compile?
The one that does compile
object First {
class ABC(body: => Unit) {
val a = 1
val b = 2
println(body)
}
def main(args: Array[String]): Unit = {
val x = new ABC {
a + b
}
}
}
This one doesn't compile on Scala 2.11 and 2.12
object Second {
class ABC(body: => Int) {
val a = 1
val b = 2
println(body)
}
def main(args: Array[String]): Unit = {
val x = new ABC {
a + b
}
}
}
It's not strange at all. Let's look at the first example:
You declare your class ABC to receive a pass by name parameter that returns Unit and you think this snippet:
val x = new ABC {
a + b
}
is passing that body parameter, it isn't.What's really happening is:
val x = new ABC(()) { a + b }
If you run that code you will see that println(body) prints () because you're not passing a value for your body parameter, the compiler allows it to compile because as the scaladoc states there is only 1 value of type Unit:
Unit is a subtype of scala.AnyVal. There is only one value of type Unit, (), and it is not represented by any object in the underlying runtime system. A method with return type Unit is analogous to a Java method which is declared void.
Since there is only one value the compiler allows you to omit it and it will fill in the gap. This doesn't happen with singleton objects because they don't extend AnyVal. Just has the default value for Int is 0 the default value for Unit is () and because there is only this value available the compiler accepts it.
From documentation:
If ee has some value type and the expected type is Unit, ee is converted to the expected type by embedding it in the term { ee; () }.
Singleton objects don't extend AnyVal so they don't get treated the same.
When you use syntax like:
new ABC {
// Here comes code that gets executed after the constructor code.
// Code here can returns Unit by default because a constructor always
// returns the type it is constructing.
}
You're merely adding things to the constructor body, you are not passing parameters.
The second example doesn't compile because the compiler cannot infer a default value for body: => Int thus you have to explicitly pass it.
Conclusion
Code inside brackets to a constructor is not the same as passing a parameter. It might look the same in same cases, but that's due to "magic".
You cannot pass a single argument to a constructor in curly braces, because this would be parsed as defining an anonymous class. If you want to do this, you need to enclose the curly braces in normal braces as well, like this:
new ABC({
a + b
})
As for why does compiler accept new ABC {a + b}, the explanation is a bit intricate and unexpected:
new ABC {...} is equivalent to new ABC() {...}
new ABC() can be parsed as new ABC(()) because of automatic tupling, which is a feature of the parser not mentioned in the specs, see SI-3583 Spec doesn't mention automatic tupling. The same feature casues the following code to compile without an error:
def f(a: Unit) = {}
f()
def g(a: (Int, Int)) = {}
g(0,1)
Note the call produces a warning (even your original example does):
Adaptation of argument list by inserting () has been deprecated: this is unlikely to be what you want.
The warning is produced since 2.11, see issue SI-8035 Deprecate automatic () insertion.
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 _))