__MDL__ (__Model Development Language__, or colloquially also referred to as __More Datatypes than Lisp__ and Domain/OS#AEGIS - wikipedia 
The initial development team consisted of Gerald Sussman and Carl Hewitt of the Artificial Intelligence Lab, and Chris Reeve, Bruce Daniels, and David Cressey of the Dynamic Modeling Group. Later, Stu Galley, also of the Dynamic Modeling Group, wrote the MDL documentation.
MDL was initially called ''Muddle''.
MDL provides several enhancements to classic Lisp. It supports several built-in data types, including lists, strings and arrays, and user-defined data types. It offers Thread (computing) expression evaluation and coroutines. Variable (computer science)s can carry both a local value within a scope, and a global value, for passing data between scopes. Advanced built-in functions supported interactive debugging of MDL programs, incremental Software development, and reconstruction of source programs from object programs.
Although MDL is obsolete, some of its features have been incorporated in later versions of Lisp. Gerald Sussman went on to develop the Scheme (programming language) language, in collaboration with Guy Steele, who later wrote the specifications for Common Lisp and Java (programming language). Carl Hewitt had already published the idea for the language ''Planner (programming language)'' before the MDL project began, but his subsequent thinking on Planner reflected lessons learned from building MDL. Planner concepts influenced languages such as Prolog and Smalltalk. Smalltalk and Simula, in turn, influenced Hewitt's future work on the actor model.
But the largest influence that MDL had was on the software genre of interactive fiction (IF). An IF game named Zork, sometimes called Dungeon, was first written in MDL. Later, Reeve, Daniels, Galley and other members of Dynamic Modeling went on to start Infocom, a company that produced many early Commercial software works of interactive fiction.
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