Custom Error Types Using Cats Effect and MTL

"This stands in contrast to bifunctor or polyfunctor techniques, which add a typed error channel within the monad itself. You can see this easily in type signatures: IO[String] indicates an IO which returns a String or may produce a Throwable error ( Future[String] is directly analogous). Something like BIO[ParseError, String] would represent a BIO that produces a String or raises a ParseError."

"In a meaningful sense, this type of bifunctor error encoding is analogous to checked exceptions in Java, whereas monofunctor error encoding (like Cats Effect's IO) is analogous to unchecked exceptions. Both are valid design decisions for an effect type, but they come with different benefits and tradeoffs. Cats has long been quite prescriptive about monofunctor effects, in part because this considerably simplifies the compositional integration space."

Cats MTL 1.6.0 provides a lightweight syntax to manage user-defined typed error channels within the Cats ecosystem without relying on complex monad transformers. The Cats Effect IO type and Cats generic typeclasses historically expose only Throwable as the error channel, unlike bifunctor or polyfunctor approaches that embed typed errors into the monad. Bifunctor encodings like BIO[ParseError, String] allow explicit user-specified error types and resemble checked exceptions, while monofunctor encodings resemble unchecked exceptions. The Cats ecosystem favors monofunctor effects because they simplify compositional integration, concurrency interactions, and resource handling, yet parsing and other domains still benefit from typed error channels.

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