Registers and Quoting in Pico Lisp

By now you should have compiled and installed Pico as per the install instructions. We will start the interpreter with ./dbg. You can create a file and just copy paste the tutorial snippets and run them in the interpreter with : (load “tutorial.l”). To rerun you hit “Esc” followed by “k” to step through prior commands, hit “Enter” when you see the load command.

The car and the cdr

As in other Lisps the Contents of Address Register (CAR) and Contents of Decrement Register (CDR) is at the heart of the language. You might also want to check out Lisp on Wikipedia before you continue. Also be sure to check out the naming conventions.

Let’s try it out:

(setq *Greeting (list "Hello" "how" "are" "you" "doing?"))
(prin (car *Greeting))

Setq will put the list created with list in the variable Greeting. Now run the above code again but substitute car Greeting with cdr Greeting. As you see this is basically the key => value system of PHP.

Let’s create a simple table/2D array:

(setq *Fruits 
      (list 
       (list "green" "apple" "guava" "avocado") 
       (list "red" "cherry" "apple")))

(prin (car *Fruits))

As you can see you will get the whole first list from the last car command, not really surprising if we follow the logic from the first example, this whole list will basically be the key that was represented by “Hello” above. Try writing

(prin (car (car *Fruits)))

on the last line instead. As you see this will return “green” which is the car of the first list which in turn is the car of the whole table. These double, triple and quadruple car/cdr calls have shortcuts. Try

(prin (caar *Fruits))

instead and you will get the same result. See what you get when you try cdr, cdar, cadr, caadr and cdadr. Which combinations are they shortcuts for? Take your time to learn how these register functions applies to various list configurations, the time will be well spent because these things are used all the time. They’re everywhere.

As it happens there is a shortcut for retrieving the contents of any key even if it’s in place 100 down the line. Try substituting the last line with

(prin (assoc "red" *Fruits))

instead. As you can see the whole list with red fruits and the key (car) is returned. This is normally not what you want when you make a call like that, you don’t want the key too, only the fruits. But armed with our knowledge of how the whole car and cdr thing works we quickly do the following:

(prin (cdr (assoc "red" *Fruits)))

That’s better, we now get all red fruits, and only the fruits.

Quoting

Try this:

(set '*Greeting '("Hello" "how" "are" "you" "doing?"))
(prin (car *Greeting))

Different than above but the result is the same, it’s easy to see that setq var is just a shortcut for set ‘var. The ‘ is a shortcut for quote, everything quoted is taken literally. Variable names inside a quoted list will of course not expand into their values - except when when evaluated or passed to a family of special functions, mapcar (described below) is one of them.

Evaluation example:

(setq *Prin 'prin)
(eval '(*Prin "hello"))

However, when not evaluated:

(setq *Prin 'prin)
(prin '(*Prin "hello"))

When thinking about quoting it helps - at least for me - to think about the turing machine that accepts instructions in the form of these long paper strips that you feed into it. On the other side you get the result of the computation. When feeding the machine a variable it will of course expand into the strip it contains. When quoting though you simply feed the machine the raw/literal strip, telling it to treat it as such.

There are however exceptions, some functions work on data by reference, just like with the & in PHP. Let’s look at such an example:

(setq *Greeting '("Hello" "how" "are" "you" "doing?"))
(prinl (pop '*Greeting))
(prinl *Greeting)

In this case, as you can see the ‘ will make the pop function treat Greeting as something passed by reference. Quoted lists can of course be created dynamically and then be passed around and executed to create yet other lists that will be executed in other places in absurdity. If done properly you can in this way utilize the power of Lisp.

As it happens there is a whole category of functions that will accept a function literal (quoted list) and use that function on another list. Let’s look at a simple example:

(de getSomething (Lst R)
    (mapcar '((Element) (R Element)) Lst) )

(setq *Fruits 
      '(("green" "apple" "guava" "avocado") 
        ("red" "cherry" "apple")))

(prin (getSomething *Fruits 'car))

In this case we will retrieve all keys, basically the same functionality as the PHP function array_keys. If you do ‘cdr you will instead get the values which corresponds to array_values(). What if you want to get the first fruit in each sub-list? Yep you guessed it, just pass ‘cadr instead. Pretty dynamic isn’t it? If we return to the Turing machine analogy, this would constitute using a placeholder on a raw strip that will get it’s value when it’s time to execute the strip/list.

What’s happening here is that the function mapcar takes a function literal as it’s first parameter, the second parameter is the list the function literal will operate on, actually mapcar can accept several lists and use their contents in the function literal, however in this case we just keep it simple. Element will be each element in the list, in our case the two sub-lists. The result will be a new list whose elements are the results of each operation our function literal performs. That is why we get a list with “green” and “red” in it if we pass ‘car to getSomething.

Notice also that there is no return keyword, in Pico Lisp (and all Lisps I think) all expressions return something, hence no need for a return keyword. And the (de… keyword is the same as function in PHP, we simply define a new function to be used later. That’s all for this time, the next tutorial will probably cover more advanced list manipulation examples.

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