Bit bucket

A bit bucket is a general term for a data structure that would be capable of storing data, or that at least implements an API to allow data to be written to it – but which is never read, only written to. While mostly useless in regular programming, bit buckets are occasionally useful when programming in esoteric programming languages, with two primary uses having been seen.

One of the main uses for bit buckets is as a method of implementing control flow in languages that do not have fully-featured control flow of their own (or where the control flow is avoided where possible due to being excessively awkward to use), in languages which have some sort of equivalent to a reference that is capable of referencing multiple different variables or structures. The idea is to conditionally skip over parts of code by doing all the writing in those parts indirectly via a reference, changing the reference to point to useful memory if the code is supposed to be run, or to point to a bit bucket if the code is supposed to be skipped; although the code runs regardless, any writes it makes if it's supposed to be skipped will be written somewhere that is never read, and thus may as well not have happened. In order for this to work, the bit bucket needs to match the API of the data structure it's replacing exactly. For example, in the I/D machine Turing-completeness proof, there are two queues: the data queue that stores data, and a bit bucket that exists simply to have somewhere to put unwanted data. (The I/D machine does not have control flow which would enable the commands that normally push data to the data queue to be skipped, and thus the bit bucket is used to allow those commands to run to no effect.) This technique has also been observed in INTERCAL programming, where performing useless writes is often easier than conditionally skipping blocks of code.

The other major use of bit buckets is when writing in reversible programming languages, especially when compiling into them from non-reversible programming languages: the problem is that a reversible program can be run in reverse in order to reconstruct its entire history of execution, and that in turn means that the data stored by the program needs to be sufficient to do that reconstruction of history. However, the program often doesn't care about its own execution history, being written to execute the same way regardless of the history. As such, it is common to use bit buckets to store the information needed to reconstruct the execution history; the bit buckets are write-only while executing the program normally, because the data stored there isn't supposed to impact the program execution in any way, but allow the data to be "reverse-written" (i.e. read) when the program is run in reverse. In most cases, compiling from an irreversible programming language into a similar reversible language is primarily a matter of implementing a bit bucket and using it to store the information needed to reverse the irreversible operations of the original language.