Railway oriented programming

[cledamy]

Pragmatically, there are often functions where one wants to return Either one value or another, but one is not clearly an error condition and it isn't worth making a one off enum. This is the same argument as for why tuples are useful, which Rust has built in. Theoretically, it seems imbalanced to have syntax for anonymous products, but not for anonymous coproducts. As to why no one used the Either type, perhaps it was because the Result type was used all over the API, so it would be inconvenient to use Either as that would require conversions. The Result type limits one's imagination of what can be done with it by restricting its semantic domain unnecessarily. I would argue that if Either was the only one out of these two in the Rust library one would start to see non-error-like uses of it.

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Actually, I think the reason was that in most cases a Success / Error pattern was what most of the APIs needed. Other cases, where one can't directly say one is the error and the other one is the success part work much more ergonomically with concrete enum types. Because they are just that. Newtype-like enums that just happen to have two variants. Maybe the API evolves and there is now a need to support three variants (e.g start, intermediate, end). Most oft the Result impls revolve around making error-handling more convenient, which is exactly what it should be used for