strongweak

Definitions for transforming between types.

• strong -> weak drops invariants (e.g. going from a bounded to an unbounded numeric type)
• weak -> Maybe strong introduces invariants

This is not a Convertible library that enumerates transformations between types into a dictionary.

• A "strong" type has exactly one "weak" representation.
• Weakening a type is safe.
• Strengthening a type may fail.

There are generic derivers for generating Strengthen and Weaken instances for arbitrary data types. The Strengthen instances annotate errors extensively, telling you the datatype & record for which strengthening failed - recursively, for nested types.

This is a validation library. We don't fail on the first error -- we attempt to validate every part of a data type, and collate the errors into a list. This happens magically in the generic deriver, but if you're writing your own instances, you may want ApplicativeDo so you can use do notation. So it'll monadic, but actually everything will get checked.