[matt_kantor]

You can't in Haskell either! For example, any function could secretly call `unsafePerformIO` to cause a side effect (and that's not the only example).

I believe `const` functions in Rust are actually be guaranteed to be pure, though I haven't followed that feature closely and there may be nuances.

In most languages purity is a norm rather than enforced by static analysis. I definitely agree that it's much safer to assume that an arbitrary Haskell function is pure than it is to assume that of an arbitrary TypeScript function.

[chuckadams]

You can compile Haskell code with the -XSafe flag, and this is communicated in the package, so something like backpack can be told to load only safe modules. Still, there's probably plenty of code that's safe but not pure, but that's as good as we're likely to get.