The aim is to make a usable untyped functional language system all written in F# which allows the implementation of pure functional features like Church numbers. The work modularises as: Lexer Parser Lambda closure runtime system The lexer performs lexical analysis, the process of converting the stream of characters to words (‘tokens’). The parser then uses these tokens to find the relationship between them and determine the syntax. In other words, the parser converts the tokens to a syntax tree that can be used by the run time to give an output.