It didn't invent any of these. This is railway-oriented-programming[1], using Either, leftMap and bind/flatMap, a conversion from Either to Try, with forced rec

It didn't invent any of these. This is railway-oriented-programming[1], using Either, leftMap and bind/flatMap, a conversion from Either to Try, with forced recover calls and explicit implicit conversions via typeclasses. Other than the enforcement by the compiler, this is common in scala and f#, two other languages where raising errors is common. Railway-oriented-programming is nice because it typechecks, and because it reduces cyclomatic complexity by short-circuiting without forcing you to handle the error until the end of the chain. Now, it is unfamiliar to those outside the FP community, but much of the rust language seems to be unfamiliar mixes of FP and low-level optimisation techniques. With the Either type you don't even need to do that, but then errors on the left becomes convention instead of an enforced habit. The only odd part is the try macro. That should wrap the Val in a right, to preserve type information. I guess I could be convinced that Either = Error | Id [A]. [1]: https://fsharpforfunandprofit.com/rop/ https://fsharpforfunandprofit.com/rop/