ASTType.java

public interface ASTType  {
    String toStr();
    public boolean eqt(ASTType t);
    default boolean subtypeOf(ASTType t, Environment<ASTType> e) throws InterpreterError {
        return subtypeOf(t, e, new java.util.HashSet<>());
    }
    boolean subtypeOf(ASTType t, Environment<ASTType> e, java.util.Set<String> seen) throws InterpreterError;
    default ASTType computeLUB(ASTType other, Environment<ASTType> e) throws TypeCheckError, InterpreterError {
        ASTType t1 = ASTTypeDef.unfold(this, e);
        ASTType t2 = ASTTypeDef.unfold(other, e);

        if (t1.subtypeOf(t2, e)) return t2;
        if (t2.subtypeOf(t1, e)) return t1;

        if (t2 instanceof ASTTEnum) {
            return t2.computeLUB(t1, e);
        }

        throw new TypeCheckError("Incompatible types: " + t1.toStr() + " vs " + t2.toStr());
    }
}