if-else in bAdkOde

I was talking to my CSE 200 (my first CS class at ASU) professor Richard Whitehouse about bAdkOde and he (rightly) pointed out that it didn’t have an explicit selection statement. He also said that unless I wanted it to be really ugly I’d need to have a selection statement. However, since I was going for ugly I figured that I’d just emulate the operation of an if and an if-else with the existing while statement.

If you know assembly, then you know that a while is simply a set of statements wrapped with a conditional branch at the top and a backwards branch at the bottom (or in other words, an if with a goto at the end. A do-while is simply a set of statements with a conditional branch (at the bottom) that branches to the top of the loop. In fact, in assembly programming there really aren’t any for loops or while loops. These keywords are simply abstractions and syntactic sugar. In bAdkOde, you can implement an if with the existing while statement if you explicitly make it break out of the loop. For example, let’s say that we want to check whether the user entered the character “0”:

?a
-48a
# print a line break
"10
# if a is zero
{=a
 # print the string "zero"
 "122"101"114"111"10
 # set the a register to a non-zero value so that we can break out of the loop
 >1a
}

I knew there was a way to implement an if-else with just while statements but I didn’t remember exactly how. Then my friend and co-worker Juan reminded me that I needed two variables. In our case, we need to use two registers:

?a
-48a
# copy the value of a into b
>ab
"10
# if a is zero
{=a
 # print the string "zero"
 "122"101"114"111"10
 # set the a register to a non-zero value so that we can break out of the loop
 >1a
}
# if the top loop failed, it means that b (which holds the same value as a) is 
# non-zero and so we can enter this block.
# if the top loop was successful, it means that b (which holds the same value
# as a) is zero and so we won't enter this block. Hence, the second block acts
# as an 'else'
{!b
 # print the string "non-zero"
 "110"111"110"45"122"101"114"111"10
 # zero out the b register
 >0b
}

Yes, quite ugly. But that’s what I’m going for! 🙂

bAdkOde: An Esoteric Language

I’ve added another project to the projects page. It’s called bAdkOde, an interpreter for an esoteric language that I designed. The very first incarnation of bAdkOde was written in Java and I actually posted it (or made a blog entry about it) over 8 years ago. For some reason I took it down. Probably because I stopped working on it. Anyway, I redesigned the language and wrote an interpreter for it in Perl about 4 or 5 years ago. I finally got around to posting it. Check out the project page for more details. Let me know what you think.

JSTL, instanceof, and hasProperty

I’ve been doing a little bit of JSTL over the past week, especially custom tags. I’ve written custom tags in Grails before, and there you use actual Groovy code. I guess this was how custom tags used to be written (in Java), but now you can can build your own custom tags using the standard tag library. The standard tag library is still pretty useful when it comes to building custom tags. Since it’s not straight Java, it forces you think really hard about your logic. You don’t want to put any business or application logic in your tag, and you want to restrict everything to view or presentation logic. A side effect of it not being Java is that if you want to do anything extremely complicated, you’re probably better off writing the tag in Java (making sure that you don’t let any business logic creep in).

While writing my own custom tag, I noticed that although instanceof is a reserved word in the JSTL EL (expression language), it is not supported as an operator. The reason I wanted to use the instanceof operator is that I have an attribute that could either be a List or a Map and depending on the type, I wanted to do different things.

Another thing I was trying to do, was to inspect the incoming object to see if it had a certain property (reflection). JSTL uses reflection so that you can access the properties of an object via dot notation, if they follow the JavaBean naming-convention. However, there was no way for me to see if an object had a certain property. To solve both these problems, I wrote my own JSTL functions.
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