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Making a distinction between pure and effectful functions doesnt require any kind of effect system though.

Having a language where "func" defines a pure function and "proc" defines a procedure that can performed arbitrary side effects (as in any imperative language really) would still be really useful, I think

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> Having a language where "func" defines a pure function and "proc" defines a procedure that can performed arbitrary side effects (as in any imperative language really) would still be really useful, I think

Rust tried that in the early days, the problem is no-one can agree on exactly what side effects make a function non-pure. You pay almost all the costs of a full effect system (and even have to add an extra language keyword) but get only some of the benefits.

The definition I’ve used for my own projects is that anything that touches anything outside the function or in any way outlives the function is impure. It works pretty well for me. That is, no i/o, mutability of a function-local variable is okay but no touching other memory state (and that variable cannot outlive the return), the same function on the same input always produces the same output, and there’s no calling of impure code from within pure code. Notice this makes closures and currying impure unless done explicitly during function instantiation, making those things at least nominally part of the input syntactically. YMMV.

nim does that. and they are called that.

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