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MUMPS is a language intended for and designed to build database applications. Secondary language features were included to help programmers make applications using minimal computing resources. The original implementations were interpreted, though modern implementations may be fully or partially compiled.

The most outstanding, and unusual, design feature of MUMPS is that database interaction is transparently built into the language. The MUMPS language assumes the presence of a MUMPS hierarchical database, which is implicitly "opened" for every application. All variable names prefixed with the caret character ("^") use permanent (instead of RAM) storage, will maintain their values after the application ends, and will be visible to (and modifiable by) other running applications. Variables using permanent storage are called Globals in MUMPS, not to be confused with the C term for unscoped variables.

Additionally, all variables (both RAM and disk-based) are hierarchical. They can all have child nodes (called subscripts in MUMPS terminology). Thus, the variable 'Car' can have subscripts "Door", "Steering Wheel" and "Engine", each of which can contain a value and have subscripts of their own. Thus, you could say to modify a nested child node of ^Car. In MUMPS terminology, "Color" is the 2nd subscript of the variable ^Car (both the names of the child-nodes and the child-nodes themselves are called subscripts). Hierarchical variables are similar to objects with properties in Object Oriented languages. Additionally, all subscripts of variables are automatically kept in sorted order. Numeric subscripts (including floating-point numbers) are stored from lowest to highest. All non-numeric subscripts are stored in alphabetical order following the numbers. In MUMPS terminology, this is canonical order. By using only non-negative integer subscripts, the MUMPS programmer can emulate the Arrays data type from other languages. Although MUMPS does not natively offer a full set of DBMS features, several DBMS systems have been built on top of it that provide application developers with flat-file, relational and network database features.

As a secondary language feature, you can abbreviate nearly all commands and native functions to a single character to save space; this was a common feature of languages designed in this period (eg, early BASICs). Additionally, there are built-in operators which treat a delimited string (eg, comma-separated values) as an array. Early MUMPS programmers would often store a structure of related information as a delimited string, parsing it after it was read in; this saved disk access time and offered considerable speed advantages on some hardware.

MUMPS has no data types. Numbers can be treated as strings of digits, or strings can be treated as numbers by numeric operators (coerced, in MUMPS terminology). Coercion can have some odd side effects, however. For example, when a string is coerced, the parser turns as much of the string (starting from the left) into a number as it can, then discards the rest (as in PHP). Thus the statement IF 20<"30 DUCKS" is evaluated as TRUE in MUMPS.

Other features of the language are intended to help MUMPS applications interact with each other in a multi-user environment. Database locks, process identifiers, and atomicity of database update transactions are all required of standard MUMPS implementations.

In contrast to languages in the C or Wirth traditions, some space characters between MUMPS statements are significant. A single space separates a command from its argument, and a space, or newline, separates each argument from the next MUMPS token. Commands which take no arguments (eg, ELSE) require two following spaces. The concept is that one space separates the command from the (nonexistent) argument, the next separates the "argument" from the next command. Newlines are also significant; an IF, ELSE or FOR command processes (or skips) everything else til the end-of-line. To make those statements control multiple lines, you must use the DO command to create a code block.

MUMPS
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