Using local variables in function built with haxe macro - macros

Question

I have a LangBuilder macro class; it's used to build a langObjects:Map<String, Dynamic> of texts in various languages at compile time, and inject this structure in classes via #:build macro. Every item of the Map has a field for every language supported. So the result is:
#:build(LangBuilder.build())
class Lang{}
trace(Lang.langObjects["hello-world"].en); //outputs "Hello World!"
trace(Lang.langObjects["hello-world"].it); //outputs "Ciao Mondo!"
This works perfectly, but I thought I could make a cleaner job hiding the langObjects structure using a function getLangText with arguments the id of the text (e.g. "hello-world") and the language code (e.g. "it").
So I'm trying to add this function to classes:
public static function getLangText(id:String, lang:String)
Its non-macro version could be expressed as:
public static function getLangText(id:String, lang:String){
var _langObj_id = langObjects[id];
switch(lang){
case "it":
return _langObj_id.it;
case "en":
return _langObj_id.en;
}
return "Missing Translation";
If i translate this function as a macro with this code:
var code = macro {
var _langObj_id = langObjects[$i{"id"}];
switch($i{"lang"}){
case "it":
return _langObj_id.it;
case "en":
return _langObj_id.en;
}
return "Missing translation";
};
var myFunc:Function = {
args: [{
name: "id",
type: TPath({name: "String", pack: []}),
meta: null
},
{
name: "lang",
type: TPath({name: "String", pack: []}),
meta: null
}],
ret: (macro:String),
expr: macro $code
};
fields.push({
pos: Context.currentPos(),
name: "getLangText",
meta: null,
kind: FieldType.FFun(myFunc),
doc: null,
access: [Access.APublic, Access.AStatic]
});
... it works without problems. However I would like to know how it could be written without the switch, to make it more flexible and to learn something about haxe macros. I have seen some examples where fields could be accessed in macros with $p{} or with object.$fieldName. However the haxe manual warns that the second form could be used only for simple identifiers; for example object.${fieldName} would not work.
So I try this code:
var code = macro {
var l:String = $i{"lang"};
var _langObj_id = langObjects[$i{"id"}];
return _langObj_id.$l;
};
The compiler gives an error
Unknown identifier : l
on the line containing return _langObj_id.$l;.
Then i tried to use the $p{} reification:
var code = macro {
var _langObj_id = langObjects[$i{"id"}];
return macro $p{["_langObj_id", $i{"lang"}]};
};
But the error is similar:
Unknown identifier : lang
I can surely change the langObjects structure to Map<String, Map<String, String>> and then change the code to:
var code = macro {
return macro langObjects[$i{"id"}][$i{"lang"}];
};
I think this would work, but now i'm trying to understand why both _langObj_id.$lang and $p{["_langObj_id", $i{"lang"}]} wouldn't work, and what would be the correct way to access a field in a situation like that.

Answer 1

The value of the lang parameter is not known at compile- / macro-time, so I don't see how you could generate a field access expression like langObjects["mytext"].en. At runtime when getLangText() is actually called, lang could be "en", or anything else. So that would still require a switch-case, if-else-chain or reflection to handle all the possible values.
If instead of using being created by a build macro, getLangText() was an expression macro / a macro function, the function call would be evaluated at compile-time, and be replaced with the expression it returns. That would allow you to generate the appropriate field access expression based on the parameters. It could look something like this:
class Macro {
public static var langObjects = ["mytext" => {en: "hello", de: "hallo"}];
public static macro function getLangText(id:String, lang:String) {
return macro {
var langObject = Macro.langObjects[$v{id}];
langObject.$lang;
}
}
}
class Main {
static function main() {
trace(Macro.getLangText("mytext", "en"));
trace(Macro.getLangText("mytext", "de"));
}
}
Compiles to the following on the JS target:
Main.main = function() {
var langObject = Macro.langObjects.get("mytext");
console.log("source/Main.hx:3:",langObject.en);
var langObject1 = Macro.langObjects.get("mytext");
console.log("source/Main.hx:4:",langObject1.de);
};
Perhaps that's what you're looking for? Hard to say without knowing what problem you're trying to solve.