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Comment by somat | Hacker News Reader

"TypeScript is structurally typed, which means two types with the same shape are the same type. string is string is string"

I don't speak typescript so am probably missing something obvious. but. why would you parse an email(or anything really) into a string? (or string equivalent) When parsed it will end up as a specific email object, that is, something closer to a C struct. What is the articles dance doing?

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Javascript doesn't have structs. The idea is that you have data on one hand and you have type witness about that data on the other hand. Type witness is something for the type system. But here you encounter the limits of structural typing versus nominal typing, because structural typing isn't able to witness that directly.

In sufficiently strong nominal type systems, I can hide the constructor for an EmailAddress type (as in: nobody can just construct an EmailAddress type). In Haskell speak, I can then export a function parseEmailAddress = rawString :: string -> EmailAddress. The function parseEmailAddress is the only place that has access to the constructor. Which means that the only way to turn a string into an EmailAddress is by calling parseEmailAddress.

Note that at runtime EmailAddress is just a string. The boundaries live in the type system, not on the value level. A structural typing system (as in TypeScript) does not enable that, it forces you to turn EmailAddress into something else than just a string.

Are you confusing Email vs EmailAddress? I think that in many cases people would prefer EmailAddress to be represented as a dumb string at runtime. But if you don't, you will easily find other examples where you have 2 structurally similar types, that you don't want to mix up.

Javascript does have structs, it calls them objects.

If I parsed an emailAddress the thing that came out it would look like {'domain':'example.com', 'user':'john-doe'} or emailaddr.domain emailaddr.user and a emailaddr.address method if you like that form. Even if what I parsed ended up as a single string-like field, I would still name that field. emailaddr.address

Salutes for the bit on hiding the constructor, that makes a lot of sense.

It probably does not help anything that in my one attempt at making a javascript web application I did not bother trying to understand how javascript likes it's objects and just forced a python looking model onto it. If any of the web development team saw my code I would definitely get laughed out of the club.

Yeah, in your example the structure is sufficiently dissimilar to a string for TypeScript not to confuse them for each other. However, if you also have an identity provider returning UserInfo objects in the form of {'domain':'example.com', 'user':'john-doe'}, you might not like it that now any email address is a valid UserInfo object. On the type level in TS, you cannot tell those types apart. But I guess you figured that out already.

In some languages you can create a type that is equivalent to a string, but it’s own distinct type (sometimes called the New Type pattern). Which I guess is the same as a struct with a single field, but languages have syntactic sugar, and depending on implementation doesn’t allocate another extra wrapper object on the heap (this would happen in JavaScript/TypeScript).

Look up NewTypes.

The article's dance is to avoid having extra fields that are completely unnecessary here. They want some kind of nominal email type, that is actually a string, so can be used in places where a string is needed, but when a method requires an "email" you can't use any string.

It's a pretty common pattern in functional programming and in many other languages nowadays

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