This work describes a new functional programming language and its implementation. Sloth is a non-strict purely functional interpreted programming language, which supports high order functions as first class values, structured data types, and pattern-matching based function definitions. Sloth was created for academical use in activities related to the use and to the implementation of functional programming languages. Therefore, there has been a great concern with the simplicity of both the language specification and its implementation. As a result, Sloth's syntax is modest, but expressive enough for practical applications. Furthermore, the great simplicity and portability of its implementation has no serious impact on its performance. The main contributions of this work are the design and implementation of Sloth, an optimization to combinator graph reduction as an alternative to the use of Turner combinators, a new architecture for the implementation of interpreted functional programming languages, based on the cooperation between two programming languages, and the performance comparison between Sloth and similar languages.