This document describes a couple of tools that help to quickly design and develop computer (formalized) languages. The first one use Flex to perform lexical analysis and the second is an extention of Prolog DCGs to perfom syntactical analysis. Initially designed as a new component for the Centaur system, these tools are now available independently and can be used to construct efficient Prolog parsers that can be integrated in Prolog or heterogeneous systems. This is the initial version of the CLF documentation. Updated version will be available online when necessary.