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    <p>Your usage is not that bad, what I find confusing is the idea that what's usually considered the configuration input for a reader (like the db connection data) is in this case the id of the user under inspection.</p> <p>First I'd change the <code>IO[I,A]</code> name to <code>Reader[I,A]</code> just for the sake of using a well known name for such operation[*]</p> <p>As for the <code>User2Colleague(u: User) : IO[UserId, DBObject]</code> method, it's not that unusual to discard the reader input to provide a constant value wrapped in the monad: this is exactly what your <code>unit</code> method does!</p> <p>In fact I'd change that to</p> <pre><code>def User2Colleague(u: User) : Reader[UserId, DBObject] = unit(coll.findOne(MongoDBObject("id" -&gt; u.colleagueId)).get) </code></pre> <p>or even more consistently with your client code usage</p> <pre><code>def User2Colleague(u: User): DBObject = coll.findOne(MongoDBObject("id" -&gt; u.colleagueId)).get ... def main(args: Array[String]) { val mng = new Mongo() val io = for { io &lt;- mng io2 &lt;- unit(DBObject2User(io)) io3 &lt;- unit(User2Colleague(io2)) } yield (DBObject2User(io3)) println(io.run(1)) </code></pre> <p>What I suggest is to write your code as <em>pure</em> as possible (i.e. with no monadic effect) unless required. This means doing your mappings in regular functions and then wrapping with <code>unit</code> only as needed (as to make composition typecheck within the for comprehension)</p> <p>[*] The usual IO monad has no input type, it just has an output type, and is usually targeted to do user I/O (console, printing, emailing, sending rockets, etc), i.e. anything which has side effects external to the program itself.</p>
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    1. POHow to implement a reader monad to access a database
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    1. USpagoda_5b
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    1. COI've just edit my code to add IO companion object to simplify the code.
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      1. USYann Moisan
    2. COBut now you have 2 ways to put a value in the monad, and they're not deferring to each other, which means code duplication. I'd try to generalize `unit[A]` to `unit[I,A]` and make `IO.apply` call that, or the other way round.
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    3. COI've edited to make `unit` call `IO.apply` to remove code duplication
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