Add `.filterMap(...)` on arbitraries

[TomerAberbach]

🚀 Feature Request

It would be nice if there were a .filterMap(...) on arbitraries.

Motivation

Sometimes I find that I need to do some mapping in .map(...), but the mapping is fallible because it's not trivial to represent all the necessary preconditions in the source arbitrary.

In that case, I end up needing to extract the precondition checks to a .filter(...), but that's annoying because it forces me to split the logic, which often results in duplicative work between the .filter(...) and .map(...).

It would be nice if I could use a .filterMap(...) and return the mapped value or null (or maybe a better API would be some Result<T> return type, so that null is an allowed mapped values).

Example

arb.filterMap(value => {
  const transformedValue = transform(value)
  if (someFancyCondition(transformedValue)) {
    return { ok: true, value: transformedValue }
  }
  return { ok: false }
})

[gruhn]

Are you using TypeScript >=5.5 ? Before that I also had this problem. E.g. I have some Arbitrary for DOM nodes:

type DomNode =
  | { type: 'link', text: string, href: string }
  | { type: 'bold', text: string }
  // ... more node type ...

declare const domNodeArb: fc.Arbitrary<DomNode>

Now I want to filter out link nodes and extract the href attribute:

const hrefArb = domNodeArb
  .filter(node => node.type === 'link')
  .map(node => node.href)

Before TypeScript 5.5 that would not work. In the .map the type checker had already "forgotten" that node is narrowed down to link-type nodes. But since 5.5 it works.

[TomerAberbach]

To clarify, the issue here is that in some cases knowing whether I should filter essentially requires the same work as mapping. So it'd be convenient to do the filter and mapping in the same callback so I don't have to duplicate the work

Outside of fast check the way I would usually do that is via flatMap where I return a list of one or zero items from the callback

[gruhn]

do you have an example?