FileInfo.com - The File Extensions Resource

.M File Extension

File Type 1	Mathematica Input File (Most common)
File Format	File
Common?	No
File Description	Contains functions or a set of instructions used by the Mathematica, a program for creating and running mathematical functions; saved in a text format, which can be viewed and edited with a basic text editor. Files named "init.m" are initialization files that contain settings that are loaded when the Mathematica program is opened.
Program(s) that open m files	Mac / Windows / Unix: Wolfram Research Mathematica; Edit with a text editor

File Type 2	Objective-C Implementation File
File Format	Standard File
Common?	No
File Description	Class implementation file used by programs written in Objective-C; begins with the `@implementation` directive; initializes variables and functions that can be referenced by other Objective-C source (.C) files; may also reference header (.H) files.
Program(s) that open m files	Mac: Apple Xcode; Edit with a text editor Unix: Apple complier (cc); Stepstone compiler (objcc); GNU Compiler (gcc); Edit with a text editor

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