# 60 PicoLisp Functions You Should Know - 2: Defining Variables Welcome back to the second part of the "60 PicoLisp Functions you should know" series. This time, we will discuss about variables: How to define them, how to work with them, and how to understand their internal representation. ------------------- *All examples can be executed in the PicoLisp [REPL](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop). If you want to follow along, type ``pil +`` in the console to open the REPL. Check [here](https://picolisp-blog.hashnode.dev/60-picolisp-functions-you-should-know-1-arithmetics) for further basic commands.* ------------------- ### Defining Variables Variables can be set by the command ``setq``. ``` : (setq A 3) -> 3 ``` The value of a symbol can be checked in the REPL like this: ``` : A -> 3 ``` We see that ``A`` evaluates to 3. The value of an undefined symbol is ``NIL``. ``` : B -> NIL ``` ---------------------------------- Now let's try the same with more complex symbols. Let's set a symbol ``Country`` to ``Japan``. Let's try it the naive way (spoiler: it won't work): ``` : (setq Country Japan) -> NIL : Country -> NIL ``` ``Country`` is ``NIL`` - what happened? As we learned in the [previous section](https://picolisp-blog.hashnode.dev/concepts-and-data-types), basically everything is a symbol, except for lists and numbers. For the interpreter, ``Japan`` is a symbol and can be **evalulated**. However, ``Japan`` doesn't have a value, therefore the evaluation returns ``NIL``, and ``Country`` is set to NIL, too. Now let's set the ``Country`` to the symbol ``Japan`` *itself*. For this we need **escape the evaluation** by using a single quote (``'``). Then it will take the symbol ``Japan`` "as such": ``` : (setq Country 'Japan) -> Japan : Country -> Japan : Japan -> NIL ``` This is an important difference. The value of a symbol can be returned by ``val``: ``` : (val 'Country) -> Japan : (val 'Japan) -> NIL ``` *We will come back to the ``quote``(``'``) function again when we will cover **anonymous functions** ([lambda calculus](https://en.wikipedia.org/wiki/Anonymous_function)) and other concepts of functional programming.* -------------------------- Any variable can have properties (key-value pairs), which are defined by ``put``. Now let's fill up our symbol ``Japan`` with some key-value pairs, and give it a value "JP". ``` : (put 'Japan 'language "Japanese") -> Japanese : (put 'Japan 'continent "Asia") -> Asia : (put 'Japan 'island T) -> T : (setq Japan "JP") -> "JP" ``` To check the value of a property, we will use ``get``. ``` : (get 'Japan 'language) -> "Japanese" ``` To print the variable with all it's properties, we can use the command ``show``. ``` : (show 'Japan) Japan "JP" language "Japanese" island continent "Asia" ``` ---------------------------- ### Internal representation In order to fully understand what is happening, let's apply what we learned in the [Concept and Data types](https://picolisp-blog.hashnode.dev/concepts-and-data-types) post. This is the internal representation of symbol ``Japan``: ![keyval.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1630855321522/2pZYEoQMN.png) *Note that the key "island" does not need a value cell for ``T``, because its existence already implies that it's ``T``.* Now we can see the difference between ``Japan`` and ``'Japan``: - ``Japan`` **evaluates** to the content of the ``VAL``cell, which is "JP". - ``'Japan`` returns the **pointer to the symbol** ``Japan``, i.e. the "symbol" arrow that we see at the top. Understanding this difference will be very helpful when we discuss more complex operations, for example on lists. ---------------------------- Congratulations, now you know can do basic arithmetics and define variables in PicoLisp. Let's continue with **input/output** and **loops** in the next post! ---------------------------- # Sources Cover pic: Gerd Altmann on Pixabay https://software-lab.de/doc/index.html https://software-lab.de/doc/tut.html#fun https://picolisp.com/wiki/?RosettaCodeAnalysis