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copied!<p>First, here is a working version:</p> <pre><code>module Network where data Loc = Loc deriving (Show, Eq) data Pair = Conn Loc Loc | Disconn Loc Loc deriving(Show,Eq) data NC = EmpNC | Inn Pair NC instance Eq NC where EmpNC == EmpNC = True (Inn p nc1) == nc2 = (nc_include p nc2) && (nc1 == nc2) _ == _ = False nc_include :: Pair -> NC-> Bool nc_include p EmpNC = False nc_include p1 (Inn p2 nc) = (p1 == p2) || (nc_include p1 nc) </code></pre> <p>What's the difference (besides my arbitrarily defining <code>Loc</code>)? Mainly formatting. </p> <p>I suppose what happened is this:</p> <p>You tried to write an instance of <code>Eq</code> for <code>NC</code>:</p> <pre><code>instance Eq NC where EmpNC == EmpNC = True ... </code></pre> <p>and realized the compiler didn't like this, saying it conflicted with <code>(==)</code> from the <code>Prelude</code>. However, you drew the wrong conclusions from this, hiding <code>Prelude.(==)</code> and going on. Now the compiler didn't complain, but it parsed what you did like this:</p> <pre><code>instance Eq NC where -- make NC an instance of Eq with only default definitions EmpNC == EmpNC = True -- define a new function (==) of type (NC -> NC -> Bool) ... </code></pre> <p>That's why now you can't apply <code>(==)</code> to <code>Pair</code>s, because it's type is <code>(NC -> NC -> Bool)</code>, hence the compiler tells you that it would have expected an <code>NC</code> instead of a <code>Pair</code>.</p> <p>Now, the core of the matter is that instance definitions follow whitespace formatting rules, so you must leave at least one whitespace column before the functions in your <code>instance</code> definition:</p> <pre><code>instance Eq NC where EmpNC == ... </code></pre> <p>works, while </p> <pre><code>instance Eq NC where EmpNC == ... </code></pre> <p>doesn't.</p> <p>The same is true for some other things like <code>class</code>, <code>do</code>, <code>case</code> etc.</p>

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