Check for admin permissions in Yesod using database

Question

I'm trying to implement the isAdmin function in a similar fashion to these Yesod book examples. The problem is that I want to keep my 'admin' emails in the database, but when I make a database query, isAdmin loses its purity, so I have to return Handler-something IO Bool instead of just Bool. Now, this scraps the very nice syntax sugar in hamlet that allows to do so:

$if isAdmin
    <p ...

Is there a way to still have a pure isAdmin function even if I make a call to the database (I seriously doubt that... by the way, I don't want to pass any parameters to isAdmin)? Or maybe I could still use the $if syntax with the impure function?

P.S.: Currently, I am using a hack in the handler module:

isadmin <- isAdmin

So, I can write in the hamlet corresponding to that handler:

$if isadmin

But it is not universal and I like the first method more.

"a pure isAdmin function" -- Terminology nitpicking: something that doesn't take arguments isn't a function. A better wording would be "a pure isAdmin value". If you were talking about the monadic isAdmin you actually have, you could also say "an isAdmin computation". — duplode

duplode duplode · Accepted Answer · 2015-09-14T05:07:50

N.B.: I don't actually know Yesod; this answer is based on general Haskell practice.

Is there a way to still have a pure isAdmin function even if I make a call to the database (I seriously doubt that

It is not possible indeed. Access to an external database must, in one way or another, live in IO; there is no way around it.

Currently, I am using a hack in the handler module:

isadmin <- isAdmin

So, I can write in the hamlet corresponding to that handler:

$if isadmin

That is not a hack; it is one of the standard ways of using the result from a monadic computation. In fact, in the Yesod book page you linked to, right before the isAdmin example you mentioned, there is a snippet that runs a database query and uses the result with $if in a very similar way to what you are doing now:

getBlogR :: Handler Html
getBlogR = do
    -- etc.
    entries <- runDB $ selectList [] [Desc EntryPosted]
    -- etc.
$if null entries
    -- etc.

P.S.:

N.B.: I don't want to pass any parameters to isAdmin

I wonder why you don't want to do so. Perhaps it is because your only need to check whether a single specific user is an admin. In that case, you should consider renaming your computation to isCurrentUserAdmin (or whatever makes sense in your use case). Seeing a name like isAdmin begs the question "who is an admin?", and so it is rather surprising that something with that name doesn't take any arguments.

Check for admin permissions in Yesod using database

1 Answers