Pair

Adds the given validator, and maps to the given higher-level type U.

Unlike a .validate(v).map(f)(g) invocation, during decoding the validator is run before applying the f function. If there are validation errors, decoding fails. However, the validator is then invoked again on the fully decoded value.

This is useful to create inputs/outputs for types, which are unrepresentable unless the validator's condition is met, e.g. due to preconditions in the constructor.

Adds a validator.

Note that validation is run on a fully decoded value. That is, during decoding, first the decoding functions are run, followed by validations. Hence any functions provided in subsequent .maps or .mapDecodes will be invoked before validation.