I've been looking for a solid Typescript implementation of "parse don't validate" that performs runtime parsing using semantics attached to the defined Typescript types themselves. In other words, much like attrs for Python, I want to be able to define a low/no-boilerplate type, and then register parsers for those types that will work recursively to parse my data, resulting in the specified Typescript type.

Has anyone seen or written something like this?

You define a decoder schema, and then the resulting TS type gets automatically derived for you. You can then run data through the decoder, it will err if there's a mismatch, or return a value of the inferred type otherwise.