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Any sufficiently capable type system will make it possible to implement a form of functional programming.

These advanced type systems are essentially a form of pre-compile time scripting to assert certain guarantees about user defined types. It's the same mechanism that makes C++ templates turing complete.

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In the case of TypeScript, most of the magic and crazy hacks are dependent on Template Literal Types [0], which is what allows inference based on arbitrary strings. Once you have that abstraction, you can just keep adding complexity to it.

[0] https://www.typescriptlang.org/docs/handbook/2/template-lite...

Any sufficiently advanced type system is indistinguishable from Prolog.

Note to readers: if the author’s username doesn’t make it obvious, it’s funny because is true.

If feeling adventurous, read e.g. https://lpn.swi-prolog.org/lpnpage.php?pagetype=html&pageid=...

commenting for future reference.

that's a great quote.

Thanks. The Shen language takes this to its logical conclusion (pun intended) and implements a fully Turing complete type system. Types are specified with sequents which are essentially Prolog relations using a slightly different notation.[1]

1: https://shenlanguage.org/OSM/Recursive.html

Yes! I have played with Shen before.

IIRC the type checker is literally a Prolog.

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