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Comment by zachrose | Hacker News Reader

Programming languages with built-in inversion of control by way of an effects system, like in Elm.

Could you elaborate on that a little?

Elm is a purely functional language, meaning all you write are the insides of functions that take a value and return a value. There's no imperative way to "do anything" like make an HTTP request, all you can do is return a "command" value from a function and the core libraries will do it.

This means that the core libraries define the signatures of the functions that an Elm application must implement. (All Elm apps "work the same way" in that sense.) It also means that you never have to mock or stub anything in a test. There are also great implications for performance and maintainability.

Elm compiles to JavaScript and is specific to browser applications, but there's no theoretical reason the same idea couldn't be applied to mobile apps or CLI applications or webservers or anything else. (It's just a lot of work.)

So it sounds like there's Elm the framework and Elm the language then. The language is the language and the framework gives inversion of control. So this is a bit unique in that usually frameworks are built in a language that has other uses, but how is it qualitatively different from programming in any other framework?

A little bit of both!

Elm the language is different from JavaScript, Java, Ruby, etc because there is no facility for statements or imperative APIs. It’s restricted in that sense. Frameworks in those languages still have some inversion of control, but there’s nothing stopping you from making a network request in your Celsius/Fahrenheit conversion function. (And over enough time that’s usually what happens, and you have fewer constraints with which to reason about what the code does.)

I can’t speak for languages that are similarly restricted, like Haskell, but my impression is that Elm is different because you don’t have to learn extra rule-bending mechanics to do I/O.

The Elm approach is unique, as far as I know. And while it doesn’t make for a truly all-purpose programming language, it’s a great model for developing libraries and applications on the web. I’m just a bit “surprised” there isn’t a similar thing focused on web servers or IOT or whatever else.

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