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    <p>The simplest option is to make Board an instance of <code>Bounded</code> (can be auto derived as well), and use the following helper functions:</p> <pre><code>next :: (Enum a, Bounded a) =&gt; a -&gt; a next = turn 1 prev :: (Enum a, Bounded a) =&gt; a -&gt; a prev = turn (-1) turn :: (Enum a, Bounded a) =&gt; Int -&gt; a -&gt; a turn n e = toEnum (add (fromEnum (maxBound `asTypeOf` e) + 1) (fromEnum e) n) where add mod x y = (x + y + mod) `rem` mod </code></pre> <p>Example Use:</p> <pre><code>&gt; next H2 G0 &gt; prev G0 H2 &gt; next F1 F2 </code></pre> <p>(inspired by the the thread at <a href="http://www.mail-archive.com/haskell-cafe@haskell.org/msg37258.html" rel="noreferrer">http://www.mail-archive.com/haskell-cafe@haskell.org/msg37258.html</a> ).</p> <p>If you really need to use <code>succ</code> and <code>pred</code> instead, I don't believe there is any laws regarding implementations of <code>Enum</code> such that <code>succ (succ x) /= x</code> for all <code>x</code> (even though that is how most work). Therefore you could just write a custom implementation of <code>Enum</code> for your type that exhibits the wraparound you desire:</p> <pre><code>instance Enum Board where toEnum 0 = G0 toEnum 1 = A1 ... toEnum 40 = H2 toEnum x = toEnum (x `mod` 40) fromEnum G0 = 0 fromEnum A1 = 1 ... fromEnum H2 = 40 </code></pre> <p>That is very tedious to implement though. Also, the type shouldn't also implement <code>Bounded</code> when using a circular definition of <code>Enum</code>, as that breaks a rule regarding <code>Bounded</code> that <code>succ maxBound</code> should result in a runtime error.</p>
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    1. POIs there some way to define an Enum in haskell that wraps around?
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    1. USDavid Miani
    1. USDavid Miani
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    1. POIs there some way to define an Enum in haskell that wraps around?
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    1. COI recommend against making succ maxBound = minBound; the Bounded typeclass explicitly says that succ maxBound should result in a runtime error.
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      1. USEdward Z. Yang
    2. COThis still doesn't let you do, say `[H1 .. A1]`, which should logically be equal to `[H1, T2, H2, GO, A1]`.
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      1. USAnschel Schaffer-Cohen
    3. CO@Edward: The second solution doesn't have Board an instance of Bounded, so that problem doesn't occur. I'll make that more explicit that the type cannot be Bounded when implementing Enum that way in the answer though.
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