(Likely because NixOS options tend to be how I communicate between pieces of code, and those have a good enough type system).
But actually I think, as something that you just use occasionally like in your JSON example, this could be really cool. I wouldn't wanna use it as "gradual typing" in the sense of aiming to gradually type a whole body of code I think I would be more interested in "selective typing".
Just occasionally when you get that itch of "ugh this will fail really ungracefully if the inputs are bad" then you just apply this where it's needed.
Some time ago I wanted to write a Nix configuration dsl for an application I use. My effort halted due to lack of type checking, and implementing one felt like too much effort which would never pay back.
I'll give a try.
For a long-running program, like a network service or whatever, yes absolutely runtime assertions are a poor substitute.
For an impure program, where which branches of code are hit or aren't hit depends on the result of network calls, filesystem state, user input, etc, then yes, a type-system will be more powerful.
However, nix is more-or-less pure. By fully evaluating the output, you can hit all of the non-dead code, and hit every runtime assertion every time.
For a completely pure program, runtime and compiletime might as well be the same, running it and caching the output is the exact same as compiling it, and if it fails to run and outputs an error, that's the same as it failing to compile.
I know the "more-or-less pure" is doing a lot of work there, and in reality there's a lot of bits that aren't really pure, but it's close enough that a runtime type system gives you a lot more than you'd get from one in a traditional language.
I think the core caution is this is not type-level checks. Anything this validates still needs to be eval'd. It's not a guarantee of correctness for all inputs but does look to be a fairly light (and useful) tool to make unexpected states easier for you and others to identify.
Gradual typing for a system like this is the proper thing to do. You can always write a static linter for sections you need tested and maybe eventually you'll be able to type nixpkgs as well.
Any solution to the nix typing problem NEEDS graduality. Nixpkgs is too large to make it possible to have one commit that fixes everything. Starting from scratch is not a realistic option either.
Typescript has types, meaning expressions can be typechecked and this is distinct from evaluating them. Typechecking is more complete/consistent/reliable - you can catch type errors that don't show up in a codepath that gets actually executed. E.g. an empty list can be well typed, which as far as I can see you can't do with contracts.
The font presented under NixOS + Firefox is near unreadable on this website.
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