# Introduction to Muddle

### Background

Muddle, later renamed MDL, was a programming language created by the
Dynamic Modeling group (also known as the Programming Technology
Division) at MIT.  The most famous application written in this
language is the Zork interactive fiction game.  The influental CLU
programming language was bootstrapped from Muddle.

Muddle is a dialect of Lisp, but with the distinct difference that
forms are bracketed by `<` and `>` instead of parentheses.  `(` and
`)` still denote lists and are occasionally present in the language
syntax.  The language is case sensitive, and all standard names must
be entered in upper case.  To submit an expression to the interpreter,
type the <kbd>Escape</kbd> key which echoes as `$`.

### Example

A common way to develop applications in Muddle, is to build a program
in the interpreter and write out a `SAVE` image file.  This file can
later be recalled using `RESTORE`.

To make a "hello world" application, create a file called HELLO MUD
with this content:

```
<DEFINE HELLO ()
  <PRINC "Hello, muddled world!">
  <TERPRI>
  <QUIT>>

<DEFINE SAVE-IT ()
  <COND (<=? <SAVE "LARS; HELLO SAVE"> "SAVED"> <QUIT>)>
  <HELLO>>

<SAVE-IT>
```

`HELLO` is the main program which prints a message and terminates.
`SAVE-IT` is used to create a `SAVE` file and arranges to call `HELLO`
when it's restored.

Now start `:muddle` and type `<FLOAD "HELLO MUD">` and
<kbd>Escape</kbd>.  It should look like this:

```
:muddle
MUDDLE 56 IN OPERATION.
LISTENING-AT-LEVEL 1 PROCESS 1
<FLOAD "HELLO MUD">$

:KILL
```

A `HELLO SAVE` file has been created with the `HELLO` procedure in it.
To run it, start `:muddle` again and type `<RESTORE "HELLO SAVE">`:

```
*:muddle
MUDDLE 56 IN OPERATION.
LISTENING-AT-LEVEL 1 PROCESS 1
<RESTORE "HELLO SAVE">$
Hello, muddled world!

:KILL
```

The sample program can be found in the HELLO directory.

### More Information

For more information, see these documents:

- [MDL Programming Primer](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Primer.pdf)
- [MDL Programming Language](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Language.pdf)
- [MDL Primer and Manual](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Primer_and_Manual.pdf)
- [MDL Programming Environment](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Environment.pdf)