3. A Programming Language

Q is an interactive programming language in the tradition of Lisp and APL:

Q1> 3+4 # This is a comment 7

The interactive prompt Q1> is used to indicate user input; the next line is the output.

Alternatively:

3+4 ==> 7

means that 3+4 evaluates to 7.

Q has APL-like features for manipulating arrays:

(1 upto 4) + [40 30 20 10] ==> [41 32 23 14]

([] is used to construct one-dimensional arrays.) Q is based on one-dimensional sequences, in contrast to APL's multi-dimensional arrays. (Arrays are viewed as nested sequences.)

A colon before an identifier indicates a declaration:

Q3> :a=[10 20] Q4> a+a 20 40

In Q3, a is declared as a new variable. Note that the = is not assignment, it is unification (bi-directional pattern matching, as in logic programming):

Q5> [:x :y] = a Q6> sprintf "x is %s, y is %s.\n" x y x is 10, y is 20.

The builtin function sprintf is similar to the C function (except that its arguments are dynamically typed, and the output is to a dynamically allocated string); printf writes to a file. Either supports many of the features of Common Lisp's format.

String literals can contain C-style escapes (such as \n). They can also contain an expression following a $, which cancels one level of quotation. If a has the value "5" then "$(3+a)" evaluates to the string "8".

3.1 Functions

Function definitions look like the following:

Q7> :(Factorial :x) = if x=0 => 1 || x * (Factorial x-1) Q8> 2 / Factorial 20 1/1216451004088320000

Here, if cond => alt1 || alt2 is Q's if-then-else syntax.

Note that no parentheses or commas are needed for a function call. Such delimiters are inconvenient for interactive use.

3.2 White space

Q uses spaces to avoid parentheses. All of the operations within one word are done before the others:

3+10*10 ==> 103 3+10 * 10 ==> 130 3 + 10*10 ==> 103

Using spaces improves terseness, and is easier to scan visually than parentheses. It is also consistent with the way a shell splits a command into words before evaluation. (You can still use parentheses for grouping if you want or need to.)

New-lines separate "statements" (which are just expressions). A new-line is equivalent with `;'. Requiring an explicit statement terminator would be clumsy and error-prone in an interactive language.

Q also uses indentation to express control structure, like Python and Haskell's "off-side" rule.