Redesigned the library interface.
After experimenting with the library, it became apparent that it makes sense to
manipulate history primarily via lenses rather than plain functions. For
example, instead of a H.undo function, the same functionality can be obtained
by exposing a H.undoIndex lens such that H.undo becomes
L.modify(H.undoIndex, R.dec).
This allows undo-redo functionality to be implemented via simple generic components. For example, a generic "count down" -button, like
const CountDownButton = ({count, label}) => (
<button disabled={R.not(count)} onClick={U.doModify(count, R.dec)}>
{U.ifElse(count, U.string`${label} (${count})`, label)}
</button>
)that knows nothing about undo-redo, can be easily wired to control both undo and redo:
<CountDownButton value={U.view(H.undoIndex, history)} label="Undo"/>
<CountDownButton value={U.view(H.redoIndex, history)} label="Redo"/>The use of lenses also simplifies the library interface significantly as instead
of three exports for a property, e.g. H.present, H.setPresent, and
H.viewPresent, a single lens is exported, e.g. H.present.