[lkitching]

Any statically-typed language with generics can express that by parameterising the request type with the body type. A bodiless request is then just Request[Nothing] (or Request[Unit] if your type system doesn't have a bottom type). Accessing the headers just requires an interface which all static languages should be able to express.

[dan-robertson]

(1) note that “statically-typed language with generics” excludes a lot of statically typed languages, including C and Go (at least pre generics).

(2) this misses the meat of the question which is how to express that (eg) a GET request doesn’t come with a body and a POST request does. I suppose that you’re suggesting that one registers a url handler with a method type and that forces the handler to accept responses of a certain type. Or perhaps you are implicitly allowing for sun types (which aren’t a thing in many static type systems.)

(3) even in C++, isn’t this suggestion hard to work with. That is, isn’t it annoying to write a program which works for any request whether or not it has a body because the type of the body must be a template parameter that adds templates to the type of every method which is generic to it. But maybe that is ok or I just don’t understand C++.

[lkitching]

1) Looking at the TIOBE index, all the static languages I recognised on there are: C,C++,C#,Visual Basic,Go,Fortran,Swift,Delphi,Cobol,Rust,Scala,Typescript,Kotlin,Haskell and D. Of these C and Go are the only two that don't appear to support generics so I don't think this approach excludes a lot of static languages.

2) If you want to distinguish GET and POST requests statically then you just need a type for them e.g.

    GetRequest<TBody> implements Request<TBody> { }

if you don't need to do this then you can just add a method field and use a single type for both. Either way you don't need to use sum types so a language like Java can express it.

3) Yes you'll have to make functions that don't care about the body type generic so this approach could become unwieldy if you have a few such properties you want to track.

[xapata]

How about values restricted to identifiers currently in the database table? There's always something the type system can't do.

[jolux]

F# has a feature called type providers that make this sort of bookkeeping between the database and the code less tedious, but even if you mess it up, static typing still gives you more safety than dynamic. If your code blew up because it should have accepted an identifier it didn’t, you know that the code has not been written to handle that case and can fix it. Alternatively, you can just choose to ignore this, and do what a dynamic language does. There is nothing stopping you from being dynamic in a static language, passing everything around as a map, etc.

[dharmaturtle]

A demo of a SQL type provider in action: https://youtu.be/RK3IGYNZDPA?t=2539

It requires a bit of elbow grease to make it work with a CICD system... but it works :D

[xapata]

That's nifty.