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    <p>You might want an <a href="http://en.wikipedia.org/wiki/Algebraic_data_type" rel="noreferrer">ADT</a> that can only be constructed with golden ratio numbers then write myfun to accept that data type.</p> <p>I've assumed Integer as a base type, but you could use others (ex: Double or Float) or even be polymorphic.</p> <p>1) Make the ADT</p> <pre><code>module Golden (Gold, getGold, buildGold) where data Gold = G Integer Integer getGold :: Gold -&gt; (Integer, Integer) getGold (G x y) = (x, y) buildGold :: Integer -&gt; Integer -&gt; Maybe Gold buildGold x y | isGolden x y = Just (G x y) | otherwise = Nothing </code></pre> <p>Notice this module exports the <code>Gold</code> type but not the constructor (namely, not <code>G</code>). So the only way to get a value of type <code>Gold</code> is with <code>buildGold</code> which performs a run-time check - but only one - so the values of Gold can be used and assumed to be a golden ratio by all the consumers without checking.</p> <p>2) Use the ADT to build <code>myfun</code></p> <pre><code>myfun :: Gold -&gt; ??? myfun g = expr where (x, y) = getGold g </code></pre> <p>Now if you try to call <code>myfun</code> with a non-golden number (a value not of type <code>Gold</code>) then you will get a compile time error.</p> <p>Recap To build golden numbers <code>buildGold</code> function must be used, which forces the number to be checked.</p> <p>Notice what gets checked when! You have a compile time guarantee that <code>myfun</code>, and all other functions you want to use with <code>Gold</code>, are always provided golden ratios. The program input (from user, network, or where ever) still needs runtime checks and that's what <code>buildGold</code> provides; obviously there's never going to be a program that can promise the human won't type something undesirable.</p> <p>The alternatives given in the comments to your question are also worthy of consideration. An ADT is slightly heavy weight if all you need is a single function, <code>myfun</code>, that can fail then just have <code>myfun :: (Integer, Integer) -&gt; Maybe ???</code>.</p>
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    1. USThomas M. DuBuisson
    1. USThomas M. DuBuisson
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    1. COHave you actually tried to run it?
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      1. USkennytm
    2. COYou should export getGold and buildGold and you should export Gold without its constructors.
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      1. USsepp2k
    3. CO`data Gold = G Double Double` if you want to use a fixed type; you'll never find two `Integer`s that satisfy the golden ratio.
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      1. USPaul Kuliniewicz
 

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