substitutedfoldq

substitutedfoldq is a function commonly found in functional programming languages, particularly those influenced by Lisp or Scheme. It serves as a generalized fold, also known as a reduce or accumulate, operation that allows for a specific condition to determine whether an element is included in the accumulation process.

At its core, substitutedfoldq iterates through a collection (like a list or vector) from left to right, applying an accumulating function to each element and a running accumulator. The "substituted" aspect refers to a predicate function that is applied to each element before it is considered for accumulation. If the predicate returns true for an element, that element is then passed to the accumulating function along with the current accumulator. If the predicate returns false, the element is effectively skipped, and the accumulator remains unchanged for that iteration.

The function typically takes three arguments: a predicate function, an accumulating function, and an initial value for the accumulator. The accumulating function itself usually takes two arguments: the current accumulator value and the current element being processed. This allows for flexible operations, such as summing only even numbers in a list, concatenating strings that meet a certain length, or building a new data structure based on filtered elements. The "q" suffix in some implementations can indicate a strict or eager evaluation of the accumulator, meaning intermediate results are computed immediately, which can sometimes offer performance benefits by avoiding the creation of large, unevaluated expressions.