MUMPS, or M, operates in a line-oriented, interpreted environment. Code is executed sequentially, one line at a time. Unlike many compiled languages that have a main function or a specific starting block, an M routine is a collection of lines that can be called at different points. These entry points are defined by line labels.

A line label is a name at the beginning of a line, starting in the first column, that identifies a block of code. It's how you reference and jump to specific parts of your program. A routine can have multiple labels, allowing for different entry points into the same block of code.

Brevity is a core principle of M syntax. Most commands are abbreviated to a single character. For instance, S stands for SET, W for WRITE, and Q for QUIT. This makes the code extremely compact, but also requires careful reading.

A crucial rule in M is the use of whitespace. The first space after a command separates the command from its arguments. Subsequent arguments on the same line are separated by commas. M has no reserved words; the language determines whether a token is a command, a variable, or a label based on its position on the line. This context-sensitive nature is a stark departure from most modern programming languages.

In the example above, ROUTINE1 is the name of the routine. START is a line label that marks an entry point. The S command assigns values to variables X, Y, and Z. The W command prints text and variable values to the output. Notice the space immediately following S and W, which is mandatory.

M uses a unique mechanism for conditional logic called a post-conditional. Instead of a multi-line if/then/else block, you can attach a condition directly to a command. The command executes only if the condition evaluates to true.

The syntax is simple: COMMAND arguments:condition. The colon separates the command and its arguments from the truth test. If the condition is false, the command is simply skipped, and execution proceeds to the next command on the line or the next line.

Here, the first W command only runs because GRADE>=65 is true. The second W command is skipped because its condition, GRADE<65, is false. Multiple commands, each with their own post-conditional, can be chained together on a single line, separated by spaces.

Ready to test your knowledge?

Understanding these core syntactic rules is the first step to reading and writing M. The language's structure is different, but it's consistent and powerful once you grasp the basics.