ASTApp.java

public class ASTApp implements ASTNode {
    ASTNode lhs, rhs;

    public ASTApp(ASTNode l, ASTNode r) {
        lhs = l;
        rhs = r;
    }

    @Override
	public IValue eval(Environment<IValue> e) throws InterpreterError
    {
        IValue fun = lhs.eval(e);
        if (fun instanceof VClosure clo) {
            IValue arg = rhs.eval(e);
            return clo.call(arg);
        } else {
            throw new InterpreterError("app: closure expected, found " + fun);
        }
    }

    @Override
    public ASTType typecheck(Environment<ASTType> e) throws TypeCheckError, InterpreterError {
        ASTType funType = lhs.typecheck(e);
        ASTType argType = rhs.typecheck(e);
        
        if (!(funType instanceof ASTTArrow arrow)) {
            throw new TypeCheckError("app: not a function, got " + funType.toStr());
        }
        
        // C <: A
        if (!argType.subtypeOf(arrow.getDom(), e)) {
            throw new TypeCheckError("app: argument type mismatch: expected " + arrow.getDom().toStr() + " but got " + argType.toStr());
        }
        
        return ASTTypeDef.unfold(arrow.getCodom(), e);
    }

    @Override
    public boolean isFreeOutsideLambda(String id) {
        return lhs.isFreeOutsideLambda(id) || rhs.isFreeOutsideLambda(id);
    }

}