issue_comments

If we're doing to do this, I would suggest that the right signature is xarray.map(func, *datasets, **optional_kwargs), matching Python's builtin map.

What I'd like to ensure is a clean separation between the arguments of xarray.map and func. Map has three "own" parameters, like func, datasets and keep_attrs. By using the **kwargs approach we are excluding these parameter names from func. Not saying that is likely that anyone would apply a function with such parameter names. But not impossible either. Also having a real dict for keyword args (and maybe a list for positional arguments of func) is more explicit.

In my implementation the order of parameters is datasets and then func to match that of Dataset.map with the implicit self. But probably func and then the datasets is more intuitive.

* Are there many other cases outside of `xr.dot` which only operate on `DataArray`s? If not, we could update that function to take a `Dataset`

I think it would be a good idea to extend dot to Datasets. However a user may wish to map a custom DataArray function to Dataset.

* Maybe jumping ahead — are there functions where the result of `func(ds1, ds2)` shouldn't be that function mapped over the matching variables?

Not sure of the context of this. In the most general case one can certainly implement any function on ds1 and ds2. Or are you referring to the built-ins such as .dot?

Could I ask what the common use cases for this would be? If I understand correctly, running map(x, y, lambda x: x + y) is equivalent to x + y.

The motivating use case was that I wanted to compute the dot-product of two DataSets (=all of their matching variables). But in general any other function which is not as simple as x + y could be used here.