would there be any merit to a FromString type class as well?

I am not sure. For one, if it's needed, we can add it later without making a breaking change. For two, I'm not sure what situations may arise where it would be desirable to do that. Also, a FromString that works on Boolean/Ordering can be defined in PureScript without a huge performance hit (unlike Int's FromString), so I don't think it needs compiler support.

Isn't FromString just the functional dependency run backwards?

Isn't FromString just the functional dependency run backwards?

Yes.

Isn't FromString just the functional dependency run backwards?

Actually, no, because an integer could be rendered in a couple of different ways. In PureScript syntax, we support both 123 and 0x0aF as valid integers. Should a type-level conversion also support that?

In other words, should ToString actually be something like this?

-- another compiler kind
data IntBase
foreign import data Base10 :: IntBase
foreign import data Base16:: IntBase

class ToString :: Int -> Base -> Symbol -> Constraint
class ToString int base string | int base -> string, string base -> int

-- compiler solved instances
instance ToString 123 Base10 "123"
instance ToString 123 Base16 "0x7B"

I've spent some time on updating this PR. However, there's a number of factors to consider. I'm going to open an issue where this can be discussed further.

Ha, there's already an issue. I'll further discuss there.

Per the discussion in #4263, I believe this is ready for a final review.

On it!

IntToString.purs passes locally.