Combinatory logic and its combinators BCKWI are a foundation for tacit (point-free) programming. But what does each letter mean? Informally, B stands for composing, C for swapping, K for discarding, W for duplicating and I is the identity function. However, B does more than that: depending on where it appears in the expression, it can also call functions or defer values for later. To help tell these purposes apart, we relate combinatory logic to the concatenative calculus of Kerby, which features separate primitives for the different facets of B.
We provide translations from combinatory logic to concatenative calculus and vice versa. In both directions we have contributions. From concatenative to combinatory, we show that first-order stack programs map to regular combinators, and which patterns of combinators higher-order stack programs map to. From combinatory to concatenative, we extend an algorithm of Kerby to make it compatible with a call-by-value evaluation order, in addition to call-by-name.
To enforce a strong association between the two worlds, our translations are simulations. That is, one evaluation step in the original program is simulated by zero or more evaluation steps in the translated program.