Why are toJSString/fromJSString in ghcjs-dom and not ghcjs-base?

[qrilka]

Isn't JSString just a javascript string? How are functions to convert it to/from String related to DOM?

[hamishmack]

The string conversion stuff in ghcjs-dom predates the ones toJSString/fromJSString in ghcjs-base. We should investigate migrating. They are subtly different though. For instance in ghcjs-dom they use toJSVal and fromJSVal (from ghcjs-base) to do the conversion. In ghcjs-base toJString/fromJSString can convert differently from toJSVal/fromJSVal for the same type. I think I prefer the ghcjs-dom approach (but could be convinced otherwise with an example of where this is useful).

Isn't JSString just a javascript string?

Yes it is. There is a bit of a wrinkle when using the old ghcjs-dom-webkit where DOMString is not a JSString (former is a C UTF8 string for use with the webkit_dom_* functions and latter is a JSStringRef for use with JavaScriptCore), but that version of ghcjs-dom should probably be considered deprecated in favour of ghcjs-dom-jsaddle.

How are functions to convert it to/from String related to DOM?

In general you will see ghcjs-dom (which is a terrible name since it has a lot more than just DOM stuff in it) exporting a lot of things that are needed to provide compatibility between the GHC and GHCJS versions of ghcjs-dom. Over time we have been trying to simply export ghcjs-base functions were possible when compiling with GHCJS. Migrating to the ghcjs-base versions of these functions would be in keeping with that effort.

[hamishmack]

Hmm just looked in ghcjs-base and it looks like we disabled it in GHCJS.Foreign. I don't think @luite liked the way it worked and I don't think he was happy with they way ghcjs-dom does it either. I think the his main concern was that user would get error messages from GHC like:

Int does not have an instance of FromJSString you could try adding "instance FromJSString Int"

New users might try that and and because Int has a FromJSVal instance it would compile, but break horribly at runtime.

I think if we can come up with a solution that makes it harder to add new instances (perhaps just removing the default implementations) we could get it into ghcjs-base.