How to create a minimal language?

[SchrodingerZhu]

So I tried to create a language with only expressions 1+1 as a trial:

class Nothing {}

@TruffleLanguage.Registration(
    id = Language.ID,
    name = "testlang", 
    defaultMimeType = Language.MIME_TYPE, 
    characterMimeTypes = Language.MIME_TYPE)
public class Language extends TruffleLanguage<Nothing>{
    public static final String ID = "testlang";
    public static final String MIME_TYPE = "application/x-testlang";

    @Override
    protected Nothing createContext(Env env) {
        return new Nothing();
    }

    
    @Override
    protected CallTarget parse(ParsingRequest request) {
        Source source = request.getSource();
        var code = source.getCharacters().toString();
        var root = new Parser(code).parse();
        return Truffle.getRuntime().createCallTarget(root);
    }
}

and

public class Main {
    private static String TEST_LANG = "testlang";
    public static void main(String[] args) {
        Source src;
        
        try {
            src = Source.newBuilder(TEST_LANG, new InputStreamReader(System.in), "<stdin>")
                .build();
        } catch (IOException ex) {
            System.err.println(ex.getMessage());
            return;
        }

        try (Context context = Context.newBuilder(TEST_LANG)
                .in(System.in)
                .out(System.out)
                .build()) {
            Value result = context.eval(src);
            if (result != null) {
                System.out.println(result);
            }
        } catch (PolyglotException ex) {
            System.err.println(ex.getMessage());
        }
    }
}

The graalvm is invoked with

/usr/lib/jvm/java-11-graalvm/bin/java -Dtruffle.class.path.append=target/untitled2-1.0-SNAPSHOT.jar -javaagent:/opt/intellij-idea-ultimate-edition/lib/idea_rt.jar=39393:/opt/intellij-idea-ultimate-edition/bin -Dfile.encoding=UTF-8 -classpath /home/schrodinger/IdeaProjects/untitled2/target/classes:/home/schrodinger/.m2/repository/org/graalvm/truffle/truffle-api/21.0.0/truffle-api-21.0.0.jar:/home/schrodinger/.m2/repository/org/graalvm/sdk/graal-sdk/21.0.0/graal-sdk-21.0.0.jar:/home/schrodinger/.m2/repository/org/graalvm/truffle/truffle-dsl-processor/21.0.0/truffle-dsl-processor-21.0.0.jar testlang.Main

However, with the above commands, I get a nullpointerexception caused by

// package com.oracle.truffle.polyglot;
public Object getPolyglotEngine(Object polyglotLanguageInstance) {
    return ((PolyglotLanguageInstance) polyglotLanguageInstance).language.engine;
}

Actually, polyglotLanguageInstance is a null value.

I cannot find more details why this is null.

[SchrodingerZhu]

The parser returns correctly with a

public class Toplevel extends RootNode {

    Expression expr;
    public Toplevel(TruffleLanguage<?> language, Expression mainExpr) {
        super(language);
        expr = mainExpr;
    }
    
    @Override
    public Object execute(VirtualFrame frame) {
        var result = expr.executeGeneric(frame);
        System.out.println(result);
        return 0;
    }
    
}

[SchrodingerZhu]

Ha, I see the reason, I forget to pass the language instance to Toplevel