The main objective of this project is to develop the theory and practice necessary to realize a programming language designer's workbench based on logic programming and action semantics. With the workbench, language designers can start a language design from scratch, actually designing and implementing a programming language in one setting. Landin's and Tennent's design principles of language design can be applied and we can examine the language by actually executing programs. Thus, the language designers can actually confirm that the designed language is in fact the one he intended. This consistency check is indeed important because it eliminates potential design errors and possible misunderstanding among designers and implementors.