[quickthrower2]

I thought of input validation for web forms. Similar thing I guess. In Haskell you can create a type that you know is a validated email address but you still need a validation function from String -> Maybe Email to actually validate it at runtime

[WJW]

I think for the usecase GP gives it'd be even better to have a function `String -> Either (LineNumber,String,[Problem]) Email`, so that you can report back which of the lines had problems and what kind of problems. For web form validation you can skip the line number but it'd still be useful to keep the list of problems, so that you can report back to the user what about their input did not conform to expectations.

[CuriousCosmic]

That's just a parser though. Like described in the post, parsers sometimes can fail but importantly they always pass along the result if they succeed. Validation functions on the other hand only validate that said data is valid.

The argument is that if you need to interact with or operate on some data you shouldn't be designing functions to validate the data but rather to render it into a useful output with well defined behaviour.