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POParameterized Maybe monad
 

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    <p>I'm making a mess with Monad, ReaderT, ... to perform a "simple?" behavior.</p> <p>I want to intersperse a test function into <code>Maybe</code> transformation (<code>Maybe</code> or another personalized monad).</p> <p>Exactly, I want avoid <code>t</code> calls, creating some kind of monad (monad, I think) on</p> <pre><code>doCalculus :: (Int -&gt; Bool) -&gt; Int -&gt; Maybe Int doCalculus f a = do b &lt;- t $ a + 1 c &lt;- t $ 2 * b d &lt;- t $ a + b + c return d where t = if f n then Just n else Nothing </code></pre> <p>example</p> <pre><code>test :: Int -&gt; Bool test n = not (n `elem` [3, 7, 9]) *Main&gt; doCalculus test 2 Nothing *Main&gt; doCalculus test 3 Just 15 *Main&gt; </code></pre> <p>I'm trying to do some monad like <code>ReaderT</code> to perform some like</p> <pre><code>runMaybeTest doCalculus test </code></pre> <p>to use as</p> <pre><code>doCalculus :: Int -&gt; Maybe' Int doCalculus a = do b &lt;- a + 1 c &lt;- 2 * b d &lt;- a + b + c return d perform = runMaybe' doCalculus test </code></pre> <p>but I can't.</p> <p>(Of course, <code>Int</code> type would be generic into monad)</p> <p>Thank you for any tip!</p> <p><strong>=== UPDATE 1 ===</strong></p> <p>I can do it! :) ... but isn't practical (I think) :(</p> <p>I had adapted a fantastic <a href="http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros" rel="nofollow">Eric Kidd post</a></p> <pre><code>import Prelude hiding (Just, Nothing, return, (&gt;&gt;=)) class Tester a where test :: a -&gt; Bool test _ = True data MMaybe a = Nothing | Just a deriving (Eq, Show) class Monad1 m a where return :: a -&gt; m a fail :: String -&gt; m a class (Monad1 m a, Monad1 m b) =&gt; Monad2 m a b where (&gt;&gt;=) :: m a -&gt; (a -&gt; m b) -&gt; m b instance (Tester a) =&gt; Monad1 MMaybe a where return = Just fail _ = Nothing instance (Tester a, Tester b) =&gt; Monad2 MMaybe a b where Nothing &gt;&gt;= _ = Nothing (Just x) &gt;&gt;= f = if test x then f x else Nothing instance Tester Int where test n = not $ n `mod` 2 == 0 &amp;&amp; n `mod` 3 == 0 test1 :: Int -&gt; MMaybe Int test1 n = return n &gt;&gt;= \a -&gt; return (a + 3) &gt;&gt;= \b -&gt; return (a + b) test2 = map test1 [1..20] </code></pre> <p>possible (important) problems are:</p> <ol> <li>is a usable monad?</li> <li>where is do notation?</li> <li>only works defining test function into a unique type (new testing functions require new types)</li> </ol> <p>but I can wrap test function into pseudo-monad... (it's something)</p>
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    1. COI'm not clear on exactly what you are trying to do(but that's not necessarily your fault). If you want to write functions with the added context of "Maybe" use the Maybe monad and chain functions using the bind >>= operator?
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    2. COIf that sounds like what you are trying to do, I will submit a complete answer and try to help.
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    3. COI don't think you'll be able to get rid of the `t` in any non-trivial way. How would you lift expressions into the monad, and how would you delimit where the tests should be performed?
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