Documentation

This provides a consistent way to convert to JSVal, with different semantics for Char. In the Char's instance of ToJS, it converts to a string instead of integer - IMHO this is less surprising.

The other reason for this class is while GHCJS base provide both IsJSVal and PToJSVal to convert to jsval, some types are instances of one or the other class. This means you can't use the "Maybe a" instance of PToJSVal if it contains IsISJVal but not pToJSVal.

This provides a consistent way to safely convert from JSVal. The semantics is that if the return value is a Just, then the JSVal is not a null value. Also, Nothing is also returned for values out of range. They are not silently truncated. (Except for Float where there may be loss of precision) during conversion.

The reason for this class is because GHCJS.Marshal.fromJSVal and GHCJS.Marshal.pFromJSVal are not safe to use as it assumes that the JSVal are of the correct type and not null. (https:/github.comghcjsghcjs-baseissues/87). The safe way to convert from JSVal is to use JavaScript.Cast or to use the 'Maybe a' instance of FromJSVal, ie fromJSVal :: JSVal -> IO (Maybe (Maybe a)), which is a bit more awkward to use, and requires IO. Also, Javascript.Cast doesn't have much instances, and it hardcodes the instance detection method to javascript isinstance which is not sufficient for complex types (https:/github.comghcjsghcjs-baseissues/86).

It is actually safe to convert from JSVal without IO because every JSVal is a copy of a value or reference. The copy never change, so the conversion will always convert to the same result/object every time.