I'm reading PureScript by Example and got to the part introducing the Reader monad. The example goes like this:
createUser :: Reader Permissions (Maybe User)
createUser = do
permissions <- ask
if hasPermission "admin" permissions
then map Just newUser
else pure Nothing
The confusing part for me is the ask function. The signature is:
ask :: forall r. Reader r r
It appears as if it creates a Reader out of thin air 😐
When I was reading about the State monad, it had the same concept with its get function. And the text explained:
the state is implemented as a function argument hidden by the State monad’s data constructor, so there is no explicit reference to pass around.
I'm guessing this is the key, and the same thing is happening here with the Reader, but I don't understand how it works...
When the above example is run via runReader, how does the provided value suddenly appear as a result of ask? The Haskell docs for ask say: Retrieves the monad environment. But my confusion is from where? The way I see it, a value gets passed to runReader, gets stored somewhere, and to get it - you call ask... but that makes no sense.
While the example is PureScript, I'm guessing any Haskell-literate person would also be able to answer, hence the Haskell tag.
I don't have a PureScript environment around currently, so I'll try to answer from a Haskell perspective, and hope it helps.
A Reader is really only a 'wrapper' around a function, so when you get a Reader r r, you really only get a reader from r to r; in other words, a function r -> r.
You can summon functions out of thin air, because, if you're a Platonist, I suppose they always exist...
When you use do notation, you're 'inside the monad', so the context r is implicit. In other words, you call a function that returns the r value, and when you use the <- arrow, you simply get that context.