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Learn X in Y mins Quick Intro

A key feature of M is that accessing local variables in memory and persistent storage use the same basic syntax, so there's no separate query language to remember. This makes it fast to program with, especially for beginners. M's syntax was designed to be concise in an era where computer memory was expensive and limited. This concise style means that a lot more fits on one screen without scrolling.

The M database is a hierarchical key-value store designed for high-throughput transaction processing. The database is organized into tree structures called "globals", which are sparse data structures with parallels to modern formats like JSON.

Originally designed in 1966 for the healthcare applications, M continues to be used widely by healthcare systems and financial institutions for high-throughput real-time applications.


Here's an example M program to calculate the Fibonacci series:

fib ; compute the first few Fibonacci terms
    new i,a,b,sum
    set (a,b)=1 ; Initial conditions
    for i=1:1 do  quit:sum>1000
    . set sum=a+b
    . write !,sum
    . set a=b,b=sum


;   Comments start with a semicolon (;)

Data Types

M has two data types:

;   Numbers - no commas, leading and trailing 0 removed.
;       Scientific notation with 'E'.
;       Floats with IEEE 754 double-precision values (15 digits of precision)
;       Examples: 20, 1e3 (stored as 1000), 0500.20 (stored as 500.2)
;   Strings - Characters enclosed in double quotes.
;       "" is the null string. Use "" within a string for "
;       Examples: "hello", "Scrooge said, ""Bah, Humbug!"""


Commands are case insensitive, and have a shortened abbreviation, often the first letter. Commands have zero or more arguments,depending on the command. M is whitespace-aware. Spaces are treated as a delimiter between commands and arguments. Each command is separated from its arguments by 1 space. Commands with zero arguments are followed by 2 spaces.


Print data to the current device.

WRITE !,"hello world" 

! is syntax for a new line. Multiple statements can be provided as additional arguments:

w !,"foo bar"," ","baz" 


Retrieve input from the user

READ var
r !,"Wherefore art thou Romeo? ",why

Multiple arguments can be passed to a read command. Constants are outputted. Variables are retrieved from the user. The terminal waits for the user to enter the first variable before displaying the second prompt.

r !,"Better one, or two? ",lorem," Better two, or three? ",ipsum


Assign a value to a variable

SET name="Benjamin Franklin"
s centi=0.01,micro=10E-6
w !,centi,!,micro



Remove a variable from memory or remove a database entry from disk.

KILL centi
k micro

Globals and Arrays

In addition to local variables, M has persistent variables stored to disk called globals. Global names must start with a caret (^). Globals are the built-in database of M.

Any variable can be an array with the assignment of a subscript. Arrays are sparse and do not have a predefined size. Arrays should be visualized like trees, where subscripts are branches and assigned values are leaves. Not all nodes in an array need to have a value.

s ^cars=20
s ^cars("Tesla",1,"Name")="Model 3"
s ^cars("Tesla",2,"Name")="Model X"
s ^cars("Tesla",2,"Doors")=5

w !,^cars 
; 20
w !,^cars("Tesla")
; null value - there's no value assigned to this node but it has children
w !,^cars("Tesla",1,"Name")
; Model 3

Arrays are automatically sorted in order. Take advantage of the built-in sorting by setting your value of interest as the last child subscript of an array rather than its value.

; A log of temperatures by date and time
s ^TEMPS("11/12","0600",32)=""
s ^TEMPS("11/12","1030",48)=""
s ^TEMPS("11/12","1400",49)=""
s ^TEMPS("11/12","1700",43)=""


; Assignment:       =
; Unary:            +   Convert a string value into a numeric value.
; Arthmetic:
;                   +   addition
­;                   -   subtraction
;                   *   multiplication
;                   /   floating-point division
;                   \   integer division
;                   #   modulo
;                   **  exponentiation
; Logical:  
;                   &   and
;                   !   or
;                   '   not
; Comparison:
;                   =   equal 
;                   '=  not equal
;                   >   greater than
;                   <   less than
;                   '>  not greater / less than or equal to
;                   '<  not less / greater than or equal to
; String operators:
;                   _   concatenate
;                   [   contains ­          a contains b 
;                   ]]  sorts after  ­      a comes after b
;                   '[  does not contain
;                   ']] does not sort after

Order of operations

Operations in M are strictly evaluated left to right. No operator has precedence over any other.
You should use parentheses to group expressions.

w 5+3*20
;You probably wanted 65
w 5+(3*20) 

Flow Control, Blocks, & Code Structure

A single M file is called a routine. Within a given routine, you can break your code up into smaller chunks with tags. The tag starts in column 1 and the commands pertaining to that tag are indented.

A tag can accept parameters and return a value, this is a function. A function is called with '$$':

; Execute the 'tag' function, which has two parameters, and write the result.
w !,$$tag^routine(a,b) 

M has an execution stack. When all levels of the stack have returned, the program ends. Levels are added to the stack with do commands and removed with quit commands.


With an argument: execute a block of code & add a level to the stack.

d ^routine    ;run a routine from the begining. 
;             ;routines are identified by a caret.
d tag         ;run a tag in the current routine
d tag^routine ;run a tag in different routine

Argumentless do: used to create blocks of code. The block is indented with a period for each level of the block:

set a=1
if a=1 do  
. write !,a
. read b
. if b > 10 d
. . w !, b 
w "hello"


Stop executing this block and return to the previous stack level.
Quit can return a value.


Clear a given variable's value for just this stack level. Useful for preventing side effects.

Putting all this together, we can create a full example of an M routine:

; RECTANGLE - a routine to deal with rectangle math
    q ; quit if a specific tag is not called

    n length,width ; New length and width so any previous value doesn't persist
    w !,"Welcome to RECTANGLE. Enter the dimensions of your rectangle."
    r !,"Length? ",length,!,"Width? ",width
    d area(length,width)            ;Do a tag
    s per=$$perimeter(length,width) ;Get the value of a function
    w !,"Perimeter: ",per

area(length,width)  ; This is a tag that accepts parameters. 
                    ; It's not a function since it quits with no value.
    w !, "Area: ",length*width
    q ; Quit: return to the previous level of the stack.

    q 2*(length+width) ; Quits with a value; thus a function

Conditionals, Looping and $Order()

F(or) loops can follow a few different patterns:

;Finite loop with counter
;f var=start:increment:stop

f i=0:5:25 w i," " ;0 5 10 15 20 25 

; Infinite loop with counter
; The counter will keep incrementing forever. Use a conditional with Quit to get out of the loop.
;f var=start:increment 

f j=1:1 w j," " i j>1E3 q ; Print 1-1000 separated by a space

;Argumentless for - infinite loop. Use a conditional with Quit.
;   Also read as "forever" - f or for followed by two spaces.
s var=""
f  s var=var_"%" w !,var i var="%%%%%%%%%%" q  
; %
; %%
; %%%
; %%%%
; %%%%%
; %%%%%%
; %%%%%%%
; %%%%%%%%
; %%%%%%%%%
; %%%%%%%%%%

I(f), E(lse), Postconditionals

M has an if/else construct for conditional evaluation, but any command can be conditionally executed without an extra if statement using a postconditional. This is a condition that occurs immediately after the command, separated with a colon (:).

; Conditional using traditional if/else
r "Enter a number: ",num
i num>100 w !,"huge"
e i num>10 w !,"big"
e w !,"small"

; Postconditionals are especially useful in a for loop.
; This is the dominant for loop construct:
;   a 'for' statement
;   that tests for a 'quit' condition with a postconditional
;   then 'do'es an indented block for each iteration

s var=""
f  s var=var_"%" q:var="%%%%%%%%%%" d  ;Read as "Quit if var equals "%%%%%%%%%%"
. w !,var

;Bonus points - the $L(ength) built-in function makes this even terser

s var=""
f  s var=var_"%" q:$L(var)>10  d  ;
. w !,var

Array Looping - $Order

As we saw in the previous example, M has built-in functions called with a single $, compared to user-defined functions called with $$. These functions have shortened abbreviations, like commands.
One of the most useful is $Order() / $O(). When given an array subscript, $O returns the next subscript in that array. When it reaches the last subscript, it returns "".

;Let's call back to our ^TEMPS global from earlier:
; A log of temperatures by date and time
s ^TEMPS("11/12","0600",32)=""
s ^TEMPS("11/12","0600",48)=""
s ^TEMPS("11/12","1400",49)=""
s ^TEMPS("11/12","1700",43)=""
; Some more
s ^TEMPS("11/16","0300",27)=""
s ^TEMPS("11/16","1130",32)=""
s ^TEMPS("11/16","1300",47)=""

;Here's a loop to print out all the dates we have temperatures for:
n date,time ; Initialize these variables with ""

; This line reads: forever; set date as the next date in ^TEMPS.
; If date was set to "", it means we're at the end, so quit.
; Do the block below
f  s date=$ORDER(^TEMPS(date)) q:date="" d
. w !,date

; Add in times too:
f  s date=$ORDER(^TEMPS(date)) q:date=""  d
. w !,"Date: ",date
. f  s time=$O(^TEMPS(date,time)) q:time=""  d
. . w !,"Time: ",time

; Build an index that sorts first by temperature - 
; what dates and times had a given temperature?
n date,time,temp
f  s date=$ORDER(^TEMPS(date)) q:date=""  d
. f  s time=$O(^TEMPS(date,time)) q:time=""  d
. . f  s temp=$O(^TEMPS(date,time,temp)) q:temp=""  d
. . . s ^TEMPINDEX(temp,date,time)=""

;This will produce a global like


There's lots more to learn about M. A great short tutorial comes from the University of Northern Iowa and Professor Kevin O'Kane's Introduction to the MUMPS Language presentation.

YottaDB and its precursor, GT.M, have thorough documentation on all the language features including database transactions, locking, and replication:
Massachusetts General Hospital Utility Multi-Programming System, or M, is a high performance transaction processing key-value database with integrated programming language. It was originally developed at Massachusetts General Hospital for managing hospital laboratory information systems. MUMPS technology has since expanded as the predominant database for health information systems and electronic health records in the United States. MUMPS-based information systems run over 40% of the hospitals in the U.S., run across all of the U.S. federal hospitals and clinics, and provide health information services for over 54% of patients across the U.S.

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Last modified 26 May 2024