# Developing and Debugging with Vip

This post builds up on the general Vip introduction, as well as on the [debugging techniques](https://picolisp-blog.hashnode.dev/levelling-up-2-use-the-debugger) from the "Getting Started" series.

In the examples, I'll be using the ``family-contemporaries.l`` script from [this post](https://picolisp-blog.hashnode.dev/creating-pdf-output). If needed, you can download the file [here](https://gitlab.com/picolisp-blog/web-applications/-/blob/main/database/family-contemporaries.l).

-------------

### The REPL ``vi`` function

In this post, we will explore how to make use of the REPL ``vi`` function, especially about its in-memory editing opportunities.

Generally, ``vi`` accepts four types of arguments:
- a **symbol** (defined with ``de``) will open the source file of this symbol. For example: ``(vi 'curry)`` will open the source code of ``curry`` in the ``lib.l`` library file.
- a **filename** will open the file: ``(vi "@lib.l")``
- a **class method + class name** will open the method definition. For example: ``(vi 'set> '+Entity)`` opens the source code of the ``set>`` method in the ``db.l`` library file.
- a **list** will edit the pretty-printed representations of the symbols (their values and properties) in a temporary file.
 
---------------

### Browsing database items

First, we start the application as usual with ``pil family-contemporaries.l -family~main -go +``. As soon as we connect with the browser, we receive a prompt in the terminal in the family namespace.

The browser window displays a database record:

![id.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1638294242139/J4k-xGnVe.png)

-----------

Now let's use Vip to browse this database item in the text editor. First let's check which item is currently displayed:

```
family: *ID
-> {A12}
```

We open this item in **memory** in Vip by calling it in a *list*:

```
: (vi '({A12}))
```

*A shortcut for in-memory editing is the ``(applicationv sym1 sym2 ...)`` command, i. e. ``(v {A12})``, without list parentheses.*

Now we can browse the item with ``K`` (forward) and ``Q`` (backwards), for example by exploring the classes or the referenced objects:


![forward.gif](https://cdn.hashnode.com/res/hashnode/image/upload/v1638294807375/OWyhveXN-.gif)


---------------

### Modifying Database Items for Testing Purposes

We cannot simply browse the objects but we can also change them. Let's replace the "job: Queen" by "King". If we refresh the browser, our new value is displayed.


![king.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1638294993537/mae3pzTiJ.png)

The change is persistent during our session, however, once we close it it's gone. This means that we can safely edit and test modified database objects without actually having to write them to the database.

**Note: Handle with care! The relations are not automatically updated, so if you accidentially commit this (for example by clicking a GUI button), your database might be corrupted.**

----------------

### Adding breakpoints and ``msg`` to the main program

Of course we're not restricted to database items. Similarly we can also modify a copy of the sourcecode in-memory, for example the ``contemporaries`` function:

```
: (v contemporaries)
```

opens the ``contemporaries`` function in memory. 

For example, let's add a breakpoint before the call of the ``html`` function:

```
contemporaries ((*ID)
   (action
      (! html
```

Then save and close it with ``:x``. Next, we press the ``contemporaries`` button and the debugger opens. We can press ``Enter`` to continue, ``e`` to evaluate, ``d`` to step into a subexpression or inspect variables by typing their name.


![debughtml.gif](https://cdn.hashnode.com/res/hashnode/image/upload/v1638301432002/2lVeRoVz2B.gif)


---------------

If you don't want to get into the debugging process so deeply, you can also simply print messages to the terminal with ``msg``.

For example, let's print out the ``(: nm)`` attribute of the current object in the ``<h2>`` in the ``person`` function, by wrapping ``msg`` around it:

```
# before
(<h2> NIL (<id> (: nm)))

# after
(<h2> NIL (<id> (msg (: nm))))
```

Now every time we load the ``person`` function, we see the current object's ``nm`` attribute printed to the terminal.


![msgDebug.gif](https://cdn.hashnode.com/res/hashnode/image/upload/v1638301951334/p9Rd1w0zY.gif)


After we close the session, breakpoint, ``msg`` and all other temporary changes are gone. This is very useful for testing and debugging. Of course you should take care to edit the original source if you want to keep the changes.

------------------

This is all for Vip for the moment - it is still evolving, so there might be updates sometime in the next year!


----------------

# Sources

https://picolisp.com/wiki/?vip   
Photo by <a href="https://unsplash.com/@cookiethepom?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText">Cookie the Pom</a> on <a href="https://unsplash.com/s/photos/computer?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText">Unsplash</a>