A compiler with some compile time type checking has been implemented starting around 1985. That's the public domain CMUCL compiler (named Python) by Robert MacL

A compiler with some compile time type checking has been implemented starting around 1985. That's the public domain CMUCL compiler (named Python) by Robert MacLachlan. See 'Types in Python' from the manual: https://common-lisp.net/project/cmucl/doc/cmu-user/compiler.html#toc127 https://common-lisp.net/project/cmucl/doc/cmu-user/compiler.... A variant of that compiler is used in the now popular SBCL implementation: http://www.sbcl.org/manual/index.html#Handling-of-Types http://www.sbcl.org/manual/index.html#Handling-of-Types