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I feel I gotta point out it's a pretty funny example—email comes up so frequently as a thing that you're recommended to neither parse nor validate, just try to send an email to the address and see if it works. If you need to know that it was received successfully, a link to click is the general method.
But "parse, don't validate" is still a generally good idea, no matter the example used. :)
I don't see it. I would much prefer to validate early rather than late. The example of 'other code might validate it differently or not at all' seems specious. I don't want invalid information "deep within the bowels of the system".
Parsing is a way of "validating early". You either get a successful parse and the program continues working on known-good data with that knowledge encoded in the type system, or you handle incorrect data as soon as it's encountered.
I understand the concept. I just disagree that it's a good idea.
Why do you think it's a bad idea? Both you and OP are in agreement that you should validate early, which seemed to be what your first comment was about. Is it encoding that the data has been validated in the typesystem that you disagree with?
I disagree that parsing is validating. For example, you could give me a valid ISO date time string, but I want a shipping date and you gave me something in the past. It parses, but is not valid.
I disagree that validating early is bad because some other part of the code might also validate later and possibly do it differently. Yes, that's bad, but not a reason to not validate early.
