R programmers can combine R and C++ to effectively navigate a variety of computing tasks: R excels as a language for interactive tasks such as data wrangling, analysis, and plotting; on the other hand, C++ can be used to efficiently carry out intensive computations. Rcpp and related tools have greatly simplified interoperability between the two languages. However, numerical computing tasks that involve functions as arguments, such as integration, root-finding, and optimization, which are routinely carried out in R, are not as straightforward in C++ within the Rcpp framework. The fntl package seeks to improve this by providing a straightforward API for numerical tools where functional arguments are specified as C++ lambda functions. Like functions in R, lambda functions can be defined in the course of a C++ program, “capturing” variables in the surrounding environment if desired, and be passed as arguments to other functions. This enables the development of R-like programs in C++, which may be appealing to Rcpp users compared to existing alternatives in the extended Rcpp family of packages. Because the overhead to evaluate a lambda function is low compared to that of evaluating an R function from C++, good performance is also possible in this paradigm.