[kaleidawave]

Ezno a semi compatible TypeScript compiler does effect tracking. It’s possible but you do have to handle the way functions handle parameters which is why it’s type system is different to the way tsc handles things. And for functions outside of the user code you have to know what effects are internally run. Things like JSON.stringify look pure until you pass it an object who has a toJSON method that does something silly. A little complicated to show what stringify internally does but possible if you add additional metadata to the definition files.

Then rather than marking a function as pure you instead check it is pure by looking at what effects its function type has!