2.1 MOO Value Types

There are only a few kinds of values that MOO programs can manipulate:

• integers (in a specific, large range)
• real numbers (represented with floating-point numbers)
• strings (of characters)
• objects (in the virtual reality)
• errors (arising during program execution)
• lists (of all of the above, including lists)

MOO supports the integers from -2^31 (that is, negative two to the power of 31) up to 2^31 - 1 (one less than two to the power of 31); that's from -2147483648 to 2147483647, enough for most purposes. In MOO programs, integers are written just as you see them here, an optional minus sign followed by a non-empty sequence of decimal digits. In particular, you may not put commas, periods, or spaces in the middle of large integers, as we sometimes do in English and other natural languages (e.g., `2,147,483,647').

Real numbers in MOO are represented as they are in almost all other programming languages, using so-called floating-point numbers. These have certain (large) limits on size and precision that make them useful for a wide range of applications. Floating-point numbers are written with an optional minus sign followed by a non-empty sequence of digits punctuated at some point with a decimal point (`.') and/or followed by a scientific-notation marker (the letter `E' or `e' followed by an optional sign and one or more digits). Here are some examples of floating-point numbers:

325.0 325. 3.25e2 0.325E3 325.E1 .0325e+4 32500e-2

All of these examples mean the same number. The third of these, as an example of scientific notation, should be read "3.25 times 10 to the power of 2".

Fine points: The MOO represents floating-point numbers using the local meaning of the C-language double type, which is almost always equivalent to IEEE 754 double precision floating point. If so, then the smallest positive floating-point number is no larger than 2.2250738585072014e-308 and the largest floating-point number is 1.7976931348623157e+308.

IEEE infinities and NaN values are not allowed in MOO. The error E_FLOAT is raised whenever an infinity would otherwise be computed; E_INVARG is raised whenever a NaN would otherwise arise. The value 0.0 is always returned on underflow.

Character strings are arbitrarily-long sequences of normal, ASCII printing characters. When written as values in a program, strings are enclosed in double-quotes, like this:

"This is a character string."

To include a double-quote in the string, precede it with a backslash (`\'), like this:

"His name was \"Leroy\", but nobody ever called him that."

Finally, to include a backslash in a string, double it:

"Some people use backslash ('\\') to mean set difference."

MOO strings may not include special ASCII characters like carriage-return, line-feed, bell, etc. The only non-printing characters allowed are spaces and tabs.

Fine point: There is a special kind of string used for representing the arbitrary bytes used in general, binary input and output. In a binary string, any byte that isn't an ASCII printing character or the space character is represented as the three-character substring "~XX", where XX is the hexadecimal representation of the byte; the input character `~' is represented by the three-character substring "~7E". This special representation is used by the functions encode_binary() and decode_binary() and by the functions notify() and read() with network connections that are in binary mode. See the descriptions of the set_connection_option(), encode_binary(), and decode_binary() functions for more details.

Objects are the backbone of the MOO database and, as such, deserve a great deal of discussion; the entire next section is devoted to them. For now, let it suffice to say that every object has a number, unique to that object. In programs, we write a reference to a particular object by putting a hash mark (`#') followed by the number, like this:

#495

Object numbers are always integers.

There are three special object numbers used for a variety of purposes: #-1, #-2, and #-3, usually referred to in the LambdaCore database as $nothing, $ambiguous_match, and $failed_match, respectively.

Errors are, by far, the least frequently used values in MOO. In the normal case, when a program attempts an operation that is erroneous for some reason (for example, trying to add a number to a character string), the server stops running the program and prints out an error message. However, it is possible for a program to stipulate that such errors should not stop execution; instead, the server should just let the value of the operation be an error value. The program can then test for such a result and take some appropriate kind of recovery action. In programs, error values are written as words beginning with `E_'. The complete list of error values, along with their associated messages, is as follows:

E_NONE No error E_TYPE Type mismatch E_DIV Division by zero E_PERM Permission denied E_PROPNF Property not found E_VERBNF Verb not found E_VARNF Variable not found E_INVIND Invalid indirection E_RECMOVE Recursive move E_MAXREC Too many verb calls E_RANGE Range error E_ARGS Incorrect number of arguments E_NACC Move refused by destination E_INVARG Invalid argument E_QUOTA Resource limit exceeded E_FLOAT Floating-point arithmetic error

The final kind of value in MOO programs is lists. A list is a sequence of arbitrary MOO values, possibly including other lists. In programs, lists are written in mathematical set notation with each of the elements written out in order, separated by commas, the whole enclosed in curly braces (`{' and `}'). For example, a list of the names of the days of the week is written like this:

{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}

Note that it doesn't matter that we put a line-break in the middle of the list. This is true in general in MOO: anywhere that a space can go, a line-break can go, with the same meaning. The only exception is inside character strings, where line-breaks are not allowed.