fix: expose url in channel interface

[hugomrdias]

Its useful to be able to get the url from the channel directly, in situations where the connection is passed into a function or class and we want to use the connection url has the default for any other communications like WS.

Instead of input url + connection we can just ask the user for connection.

example : storacha/w3up#404

[travis]

reopening, I'm going to take over getting storacha/w3up#404 merged - @Gozala would love a review from you when you get a chance!

[alanshaw]

The HTTP channel already has a url property: https://github.com/web3-storage/ucanto/blob/main/packages/transport/src/http.js#L47

I think you just need an instanceof check:

import { Channel as HttpChannel } from '@ucanto/transport/http'

const { channel } = connection
if (!(channel instanceof HttpChannel)) throw new Error('HTTP channel is required')
const { url } = channel
// do something with URL

I assume the idea is that not all channels will be HTTP in the future...

If you add the type (this PR) then you're still going to need the conditional to check for existence of the url property so you may as well just check with instanceof or duck type it.

[travis]

closing in favor of a solution to the underlying issue proposed in storacha/w3up#440