We present a programming pattern where a recursive function defined over a data structure traverses another data structure at return time. The idea is that the recursive calls get us 'there' by traversing the first data structure and the returns get us 'back again' while traversing the second data structure. We name this programming pattern of traversing a data structure at call time and another data structure at return time "There And Back Again" (TABA).The TABA pattern directly applies to computing symbolic convolutions and to multiplying polynomials. It also blends well with other programming patterns such as dynamic programming and traversing a list at double speed. We illustrate TABA and dynamic programming with Catalan numbers. We illustrate TABA and traversing a list at double speed with palindromes and we obtain a novel solution to this traditional exercise. Finally, through a variety of tree traversals, we show how to apply TABA to other data structures than lists.A TABA-based function written in direct style makes full use of an ALGOL-like control stack and needs no heap allocation. Conversely, in a TABA-based function written in continuation-passing style and recursively defined over a data structure (traversed at call time), the continuation acts as an iterator over a second data structure

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This paper introduces a sequence of λ-expressions modeling the binary expansion of integers. We derive expressions computing the test for zero, the successor function, and the predecessor function, thereby showing the sequence to be an adequate numeral system, i.e. one in which all recursive functions are lambda-definable. These functions can be computed efficiently; To this end, we introduce a notion of complexity that is independent of the order of evaluation.