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I am trying to write some externs to some Lua libraries that require to pass dictionary tables and I want to make them type safe.
So far, I have been declaring abstract classes with public inline constructors, but this gets tedious really fast:
abstract JobOpts(Table<String, Dynamic>) {
public inline function new(command:String, args:Array<String>) {
this = Table.create(null, {
command: command,
arguments: Table.create(args)
});
}
}
Is there a better way that allows me to keep things properly typed but that does not require that much boilerplate?
Please note that typedefs and anonymous structures are not valid options, because they introduce nasty fields in the created table and also do a function execution to assign a metatable to them:
--typedef X = {cmd: String}
_hx_o({__fields__={cmd=true},cmd="Yo"})
My abstract code example compiles to a clean lua table, but it is a lot of boilerplate
Some targets support #:nativeGen to strip Haxe-specific metadata from objects, but this does not seem to be the case for typedefs on Lua target. Fortunately, Haxe has a robust macro system so you can make the code write itself. Say,
Test.hx:
import lua.Table;
class Test {
public static function main() {
var q = new JobOpts("cmd", ["a", "b"]);
Sys.println(q);
}
}
#:build(TableBuilder.build())
abstract JobOpts(Table<String, Dynamic>) {
extern public inline function new(command:String, args:Array<String>) this = throw "no macro!";
}
TableBuilder.hx:
import haxe.macro.Context;
import haxe.macro.Expr;
class TableBuilder {
public static macro function build():Array<Field> {
var fields = Context.getBuildFields();
for (field in fields) {
if (field.name != "_new") continue; // look for new()
var f = switch (field.kind) { // ... that's a function
case FFun(_f): _f;
default: continue;
}
// abstract "constructors" transform `this = val;`
// into `{ var this; this = val; return this; }`
var val = switch (f.expr.expr) {
case EBlock([_decl, macro this = $x, _ret]): x;
default: continue;
}
//
var objFields:Array<ObjectField> = [];
for (arg in f.args) {
var expr = macro $i{arg.name};
if (arg.type.match(TPath({ name: "Array", pack: [] } ))) {
// if the argument's an array, make an unwrapper for it
expr = macro lua.Table.create($expr, null);
}
objFields.push({ field: arg.name, expr: expr });
}
var objExpr:Expr = { expr: EObjectDecl(objFields), pos: Context.currentPos() };
val.expr = (macro lua.Table.create(null, $objExpr)).expr;
}
return fields;
}
}
And thus...
Test.main = function()
local this1 = ({command = "cmd", args = ({"a","b"})});
local q = this1;
_G.print(Std.string(q));
end
Do note, however, that Table.create is a bit of a risky function - you will only be able to pass in array literals, not variables containing arrays. This can be remedied by making a separate "constructor" function with the same logic but without array➜Table.create unwrapping.
I am trying to make a source generator that would mimic C# anonymous objects, because they are great for when you are manipulating with collections (Select, GroupBy, etc.).
Imagine this code:
class Person {
final String firstName;
final String lastName;
final int age;
Person(this.firstName, this.age, this.lastName);
}
class TestClass {
final _data = [
Person('John', 'Doe', 51),
Person('Jane', 'Doe', 50),
Person('John', 'Smith', 40),
];
void testMethod() {
final map1 = _data.map((p) => _$$1(name: p.firstName, age: p.age));
final map2 = _data.map((p) => _$$2(fullName: '${p.firstName} ${p.lastName}', age: p.age));
}
}
Those _$$x objects are what I want to generate now. I need to somehow find them and find what is being passed into them, so my code generator would generate this:
class _$$1 {
final String name;
final int age;
const _$$1({required this.name, required this.age});
}
class _$$2 {
final String fullName;
final int age;
const _$$1({required this.fullName, required this.age});
}
but I cannot seem to even find method content:
FutureOr<String?> generate(LibraryReader library, BuildStep buildStep) {
for (final clazz in library.classes) {
final method = clazz.methods.first;
method.visitChildren(RecursiveElementVisitor<dynamic>());
}
}
it looks like the MethodElement doesn't have any children? so this doesn't look like the right way.
Is there any other way to find what I need?
A visitor can be used at the lower-level Abstract Syntax Tree to find the _$$x constructor invocations.
The visitor should also visit the whole library rather than just classes as is done in your example, in order to locate top-level usages as well.
The AST does not distinguish between constructor and method invocations, but we can use a series of checks to make sure that the invocation in question is an appropriate target for code generation nonetheless. In a similar fashion, checks can also be put in place to ensure that the invocation is done in a closure.
The following example implements this approach, and leaves you with a map of '_$$x' to MethodInvocations to work with:
FutureOr<String?> generate(LibraryReader library, BuildStep buildStep) {
final libraryElement = libraryReader.element;
final parsedLibraryResult = libraryElement.session
.getParsedLibraryByElement(libraryElement) as ParsedLibraryResult;
final libraryCompilationUnit = parsedLibraryResult.units[0].unit;
final selectorInstantiationLocator = SelectorInstantiationLocator();
libraryCompilationUnit.visitChildren(selectorInstantiationLocator);
final selectorInstantiations =
selectorInstantiationLocator.selectorInstantiations;
// ...
}
class SelectorInstantiationLocator extends RecursiveAstVisitor<void> {
final selectorInstantiations = <String, MethodInvocation>{};
#override
void visitMethodInvocation(MethodInvocation node) {
// Ensure that the invocation is an appropriate target for code generation.
// &= is not used in favour of the short-circuit && operator (https://github.com/dart-lang/language/issues/23).
// Stop if the invocation doesn't match the required prefix.
final className = node.methodName.name;
var isSelectorInstantiation = className.startsWith(r'_$$');
final classIndex = int.tryParse(className.substring(3));
isSelectorInstantiation =
isSelectorInstantiation && (classIndex != null && classIndex >= 0);
// No target will exist for a constructor invocation.
isSelectorInstantiation =
isSelectorInstantiation && node.realTarget == null;
// The selector instantiation should be done in an expression function body (=>).
isSelectorInstantiation =
isSelectorInstantiation && node.parent is ExpressionFunctionBody;
// The function body should be part of a function expression (rather than a method declaration)
isSelectorInstantiation =
isSelectorInstantiation && node.parent!.parent is FunctionExpression;
// The function expression should be inside an argument list.
isSelectorInstantiation =
isSelectorInstantiation && node.parent!.parent!.parent is ArgumentList;
if (isSelectorInstantiation) selectorInstantiations[className] = node;
return super.visitMethodInvocation(node);
}
}
The DSL I'm working on allows users to define a 'complete text substitution' variable. When parsing the code, we then need to look up the value of the variable and start parsing again from that code.
The substitution can be very simple (single constants) or entire statements or code blocks.
This is a mock grammar which I hope illustrates my point.
grammar a;
entry
: (set_variable
| print_line)*
;
set_variable
: 'SET' ID '=' STRING_CONSTANT ';'
;
print_line
: 'PRINT' ID ';'
;
STRING_CONSTANT: '\'' ('\'\'' | ~('\''))* '\'' ;
ID: [a-z][a-zA-Z0-9_]* ;
VARIABLE: '&' ID;
BLANK: [ \t\n\r]+ -> channel(HIDDEN) ;
Then the following statements executed consecutively should be valid;
SET foo = 'Hello world!';
PRINT foo;
SET bar = 'foo;'
PRINT &bar // should be interpreted as 'PRINT foo;'
SET baz = 'PRINT foo; PRINT'; // one complete statement and one incomplete statement
&baz foo; // should be interpreted as 'PRINT foo; PRINT foo;'
Any time the & variable token is discovered, we immediately switch to interpreting the value of that variable instead. As above, this can mean that you set up the code in such a way that is is invalid, full of half-statements that are only completed when the value is just right. The variables can be redefined at any point in the text.
Strictly speaking the current language definition doesn't disallow nesting &vars inside each other, but the current parsing doesn't handle this and I would not be upset if it wasn't allowed.
Currently I'm building an interpreter using a visitor, but this one I'm stuck on.
How can I build a lexer/parser/interpreter which will allow me to do this? Thanks for any help!
So I have found one solution to the issue. I think it could be better - as it potentially does a lot of array copying - but at least it works for now.
EDIT: I was wrong before, and my solution would consume ANY & that it found, including those in valid locations such as inside string constants. This seems like a better solution:
First, I extended the InputStream so that it is able to rewrite the input steam when a & is encountered. This unfortunately involves copying the array, which I can maybe resolve in the future:
MacroInputStream.java
package preprocessor;
import org.antlr.v4.runtime.ANTLRInputStream;
public class MacroInputStream extends ANTLRInputStream {
private HashMap<String, String> map;
public MacroInputStream(String s, HashMap<String, String> map) {
super(s);
this.map = map;
}
public void rewrite(int startIndex, int stopIndex, String replaceText) {
int length = stopIndex-startIndex+1;
char[] replData = replaceText.toCharArray();
if (replData.length == length) {
for (int i = 0; i < length; i++) data[startIndex+i] = replData[i];
} else {
char[] newData = new char[data.length+replData.length-length];
System.arraycopy(data, 0, newData, 0, startIndex);
System.arraycopy(replData, 0, newData, startIndex, replData.length);
System.arraycopy(data, stopIndex+1, newData, startIndex+replData.length, data.length-(stopIndex+1));
data = newData;
n = data.length;
}
}
}
Secondly, I extended the Lexer so that when a VARIABLE token is encountered, the rewrite method above is called:
MacroGrammarLexer.java
package language;
import language.DSL_GrammarLexer;
import org.antlr.v4.runtime.Token;
import java.util.HashMap;
public class MacroGrammarLexer extends MacroGrammarLexer{
private HashMap<String, String> map;
public DSL_GrammarLexerPre(MacroInputStream input, HashMap<String, String> map) {
super(input);
this.map = map;
// TODO Auto-generated constructor stub
}
private MacroInputStream getInput() {
return (MacroInputStream) _input;
}
#Override
public Token nextToken() {
Token t = super.nextToken();
if (t.getType() == VARIABLE) {
System.out.println("Encountered token " + t.getText()+" ===> rewriting!!!");
getInput().rewrite(t.getStartIndex(), t.getStopIndex(),
map.get(t.getText().substring(1)));
getInput().seek(t.getStartIndex()); // reset input stream to previous
return super.nextToken();
}
return t;
}
}
Lastly, I modified the generated parser to set the variables at the time of parsing:
DSL_GrammarParser.java
...
...
HashMap<String, String> map; // same map as before, passed as a new argument.
...
...
public final SetContext set() throws RecognitionException {
SetContext _localctx = new SetContext(_ctx, getState());
enterRule(_localctx, 130, RULE_set);
try {
enterOuterAlt(_localctx, 1);
{
String vname = null; String vval = null; // set up variables
setState(1215); match(SET);
setState(1216); vname = variable_name().getText(); // set vname
setState(1217); match(EQUALS);
setState(1218); vval = string_constant().getText(); // set vval
System.out.println("Found SET " + vname +" = " + vval+";");
map.put(vname, vval);
}
}
catch (RecognitionException re) {
_localctx.exception = re;
_errHandler.reportError(this, re);
_errHandler.recover(this, re);
}
finally {
exitRule();
}
return _localctx;
}
...
...
Unfortunately this method is final so this will make maintenance a bit more difficult, but it works for now.
The standard pattern to handling your requirements is to implement a symbol table. The simplest form is as a key:value store. In your visitor, add var declarations as encountered, and read out the values as var references are encountered.
As described, your DSL does not define a scoping requirement on the variables declared. If you do require scoped variables, then use a stack of key:value stores, pushing and popping on scope entry and exit.
See this related StackOverflow answer.
Separately, since your strings may contain commands, you can simply parse the contents as part of your initial parse. That is, expand your grammar with a rule that includes the full set of valid contents:
set_variable
: 'SET' ID '=' stringLiteral ';'
;
stringLiteral:
Quote Quote? (
( set_variable
| print_line
| VARIABLE
| ID
)
| STRING_CONSTANT // redefine without the quotes
)
Quote
;
Is there any trick to print the address of a dart object?
I'm having an issue in which I have a standard enum class as described in Does Dart support enumerations?.
class Access {
static const IA = const Access._(0);
static const RO = const Access._(1);
static const RW = const Access._(2);
final int value;
const Access._(this.value);
...
}
The variable access is of type Access and the value is 1, yet access==Access.RO is returning false.
var access = _.access;
print('''
access => ($access, ${access.runtimeType}, ${access.value})
static => (${Access.RO}, ${Access.RO.runtimeType}, ${Access.RO.value})
equal ? => ${(access == Access.RO)}
''');
prints
access => (RO, Access, 1)
static => (RO, Access, 1)
equal ? => false
If I provide an operator==(Access other) that compares the values it returns the expected value. So, I figured maybe this has to do with coming at the class from different libraries (maybe isolate related) and if I could print the address of access and Access.RO I could see if they are different. Of course, if they were different I'd then need to know why as well.
When you deal with const you have to be very careful. If you use new Access._(0) instead of const Access._(0) you will not get the same object. Here is an example :
class Access {
static const IA = const Access._(0);
static const RO = const Access._(1);
static const RW = const Access._(2);
final int value;
const Access._(this.value);
}
main(){
print(Access.IA == const Access._(0)); // prints true
print(Access.IA == new Access._(0)); // prints false
}
This could explain your problem.
If identical(a, b) returns false you can be sure that their pointers are different. (The inverse is not necessarily true for numbers, but that's an implementation detail).
If you are dealing with isolates you need to be careful when you transmit objects. Even, if they are canonicalized on one side, they won't be, after they have been transmitted to another isolate:
import 'dart:async';
import 'dart:isolate';
class A { const A(); }
foo() {
port.receive((x, _) {
print(x == const A()); // Prints false.
});
}
main() {
var p = new ReceivePort(); // To keep the program running.
spawnFunction(foo).send(const A(), null);
}
Also note that dart2js doesn't allow to transmit arbitrary objects.
I am looking for a way to create a function with a variable number of arguments or parameters in Dart. I know I could create an array parameter instead, but I would prefer to not do that because I'm working on a library where syntactic brevity is important.
For example, in plain JavaScript, we could do something like this (borrowed from here):
function superHeroes() {
for (var i = 0; i < arguments.length; i++) {
console.log("There's no stopping " + arguments[i]);
}
}
superHeroes('UberMan', 'Exceptional Woman', 'The Hunk');
However, in dart, that code will not run. Is there a way to do the same thing in dart? If not, is this something that is on the roadmap?
You can't do that for now.
I don't really know if varargs will come back - they were there some times ago but have been removed.
However it is possible to emulate varargs with Emulating functions. See the below code snippet.
typedef OnCall = dynamic Function(List arguments);
class VarargsFunction {
VarargsFunction(this._onCall);
final OnCall _onCall;
noSuchMethod(Invocation invocation) {
if (!invocation.isMethod || invocation.namedArguments.isNotEmpty)
super.noSuchMethod(invocation);
final arguments = invocation.positionalArguments;
return _onCall(arguments);
}
}
main() {
final superHeroes = VarargsFunction((arguments) {
for (final superHero in arguments) {
print("There's no stopping ${superHero}");
}
}) as dynamic;
superHeroes('UberMan', 'Exceptional Woman', 'The Hunk');
}
Dart does indirectly support var-args as long as you aren't too much into syntactic brevity.
void testFunction([List<dynamic> args=[]])
{
for(dynamic arg:args)
{
// Handle each arg...
}
}
testFunction([0, 1, 2, 3, 4, 5, 6]);
testFunction();
testFunction([0, 1, 2]);
Note: You can do the same thing with named parameters, but you'll have to handle things internally, just in case if the user (of that function; which could be you) decides to not pass any value to that named parameter.
I would like to thank #Ladicek for indirectly letting me know that a word like brevity exists in English.
This version:
Works with both positional and keyword arguments.
Supports typing of the return value.
Works with modern Dart.
typedef VarArgsCallback = void Function(List<dynamic> args, Map<String, dynamic> kwargs);
class VarArgsFunction {
final VarArgsCallback callback;
static var _offset = 'Symbol("'.length;
VarArgsFunction(this.callback);
void call() => callback([], {});
#override
dynamic noSuchMethod(Invocation inv) {
return callback(
inv.positionalArguments,
inv.namedArguments.map(
(_k, v) {
var k = _k.toString();
return MapEntry(k.substring(_offset, k.length - 2), v);
},
),
);
}
}
void main() {
dynamic myFunc = VarArgsFunction((args, kwargs) {
print('Got args: $args, kwargs: $kwargs');
});
myFunc(1, 2, x: true, y: false); // Got args: [1, 2], kwargs: {x: true, y: false}
}
Thanks, Alexandre for your answer!
I played around a little with Alexandre Ardhuin's answer and found that we can tweak a couple of things to make this work in the current version of Dart:
class VarArgsClass {
noSuchMethod(InvocationMirror invocation) {
if (invocation.memberName == 'superheroes') {
this.superheroes(invocation.positionalArguments);
}
}
void superheroes(List<String> heroNames) {
for (final superHero in heroNames) {
print("There's no stopping ${superHero}!");
}
}
}
main() {
new VarArgsClass().superheroes('UberMan', 'Exceptional Woman', 'The Hunk');
}
This has lots of problems, including:
A warning is generated wherever you call superheroes() because the signature doesn't match your parameters.
More manual checking would need to be done to make sure the list of arguments passed to superheroes is really a List<String>.
Needing to check the member name in noSuchMethod() makes it more likely you'll forget to change the 'superheroes' string if you change the method name.
Reflection makes the code path harder to trace.
BUT if you are fine with all of those issues, then this gets the job done.
If you are really into syntactic brevity, just declare a function/method with say 10 optional positional parameters and be done. It's unlikely someone will call that with more than 10 arguments.
If it sounds like a hack, that's because it is a hack. But I've seen the Dart team doing the same :-)
For example:
void someMethod(arg0, [arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9]) {
final args = [arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9];
args.removeWhere((value) => value == null);
/* do something the the args List */
print(args);
}
For the example you've written, I think you're best off using a list. Sorry about that!
I'm looking at dartbug.com, but I don't see a feature request for this. You're definitely welcome to create one!