Implement tee/tap for functors?

The generic iter takes a functor and an action to perform, and returns unit.

I’d like a similar function that passes through the functor, i.e. performs a unit-returning action and then returns the original input (for further chaining). As I understand it, this is typically called tee or tap. Could this be added?

Le me know if this is unclear and I should provide more details.

[gusty]

@cmeeren Sorry about my poor explanation, here’s a (hopefully) better attempt.

Result<'T,'U> (read 'T and 'U as variable types) is a bifunctor, as I can map a ('T -> 'A) and a ('U -> 'B) and get a "bimapped"version of it.

Now, Result<'T,unit> (unit or another fixed type) is not, because I can’t map any function to the right side, I could only map a unit->unit function, otherwise I would end up with a different type.

[gusty]

@wallymathieu I think tap is also used in Scala.

@cmeeren well, the composition is backwards map << tee but you’re right in that it won’t be efficient.

An easy alternative is let inline ftee f x = iter f x; x