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POAmbiguous type variables for dependent class constraints
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  1. POAmbiguous type variables for dependent class constraints
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    copied!<p>I'm writing a new authentication system for the <a href="http://snapframework.com/" rel="nofollow">Snap web framework</a>, because the built-in one isn't modular enough, and it has some features that are redundant/"dead weight" for my application. This problem isn't related to Snap at all, though.</p> <p>While doing so, I hit a problem with ambiguous type constraints. In the following code, it seems obvious to me that the type of <code>back</code> can only be the type variable <code>b</code> in the functions type, yet GHC complains that the type is ambiguous.</p> <p>How can I change the following code such that the type of <code>back</code> is <code>b</code>, without using e.g. <code>ScopedTypeVariables</code> (because the problem is with the constraint, not with having too general types)? Is there a functional dependency that is needed somewhere?</p> <p>Relevant type classes:</p> <pre><code>data AuthSnaplet b u = AuthSnaplet { _backend :: b , _activeUser :: Maybe u } -- data-lens-template:Data.Lens.Template.makeLens -- data-lens:Data.Lens.Common.Lens -- generates: backend :: Lens (AuthSnaplet b u) b makeLens ''AuthSnaplet -- Some encrypted password newtype Password = Password { passwordData :: ByteString } -- data-default:Data.Default.Default class Default u =&gt; AuthUser u where userLogin :: Lens u Text userPassword :: Lens u Password class AuthUser u =&gt; AuthBackend b u where save :: MonadIO m =&gt; b -&gt; u -&gt; m u lookupByLogin :: MonadIO m =&gt; b -&gt; Text -&gt; m (Maybe u) destroy :: MonadIO m =&gt; b -&gt; u -&gt; m () -- snap:Snap.Snaplet.Snaplet class AuthBackend b u =&gt; HasAuth s b u where authSnaplet :: Lens s (Snaplet (AuthSnaplet b u)) </code></pre> <p>The code that fails:</p> <pre><code>-- snap:Snap.Snaplet.with :: Lens v (Snaplet v') -&gt; m b v' a -&gt; m b v a -- data-lens-fd:Data.Lens.access :: MonadState a m =&gt; Lens a b -&gt; m b loginUser :: HasAuth s b u =&gt; Text -&gt; Text -&gt; Handler a s (Either AuthFailure u) loginUser uname passwd = with authSnaplet $ do back &lt;- access backend maybeUser &lt;- lookupByLogin back uname -- !!! type of back is ambiguous !!! -- ... For simplicity's sake, let's say the function ends like this: return . Right . fromJust $ maybeUser </code></pre> <p>Full error:</p> <pre><code>src/Snap/Snaplet/Authentication.hs:105:31: Ambiguous type variables `b0', `u0' in the constraint: (HasAuth s b0 u0) arising from a use of `authSnaplet' Probable fix: add a type signature that fixes these type variable(s) In the first argument of `with', namely `authSnaplet' In the expression: with authSnaplet In the expression: with authSnaplet $ do { back &lt;- access backend; maybeUser &lt;- lookupByLogin back uname; ... } src/Snap/Snaplet/Authentication.hs:107:16: Ambiguous type variable `b0' in the constraint: (AuthBackend b0 u) arising from a use of `lookupByLogin' Probable fix: add a type signature that fixes these type variable(s) In a stmt of a 'do' expression: maybeUser &lt;- lookupByLogin back uname In the second argument of `($)', namely `do { back &lt;- access backend; maybeUser &lt;- lookupByLogin back uname; ... }' In the expression: with authSnaplet $ do { back &lt;- access backend; maybeUser &lt;- lookupByLogin back uname; ... } </code></pre>
 

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