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    <p>There is no specific implementation for <code>IO</code>; it's an abstract type, with the exact implementation left undefined by the Haskell Report. Indeed, there's nothing stopping an implementation implementing <code>IO</code> and its <code>Monad</code> instance as compiler primitives, with no Haskell implementation at all.</p> <p>Basically, <code>Monad</code> is used as an <em>interface</em> to <code>IO</code>, which cannot itself be implemented in pure Haskell. That's probably all you need to know at this stage, and diving into implementation details is likely to just confuse, rather than give insight.</p> <p>That said, if you look at GHC's source code, you'll find that it represents <code>IO a</code> as a function looking like <code>State# RealWorld -&gt; (# State# RealWorld, a #)</code> (using an <a href="http://www.haskell.org/ghc/docs/latest/html/users_guide/primitives.html">unboxed tuple</a> as the return type), but this is misleading; it's an implementation detail, and these <code>State# RealWorld</code> values do not actually exist at runtime. <code>IO</code> is <em>not</em> a state monad,<sup>1</sup> in theory or in practice.</p> <p>Instead, GHC uses <em>impure</em> primitives to implement these IO operations; the <code>State# RealWorld</code> "values" are only to stop the compiler reordering statements by introducing data dependencies from one statement to the next.</p> <p>But if you really want to see GHC's implementation of <code>return</code> and <code>(&gt;&gt;=)</code>, here they are:</p> <pre><code>returnIO :: a -&gt; IO a returnIO x = IO $ \ s -&gt; (# s, x #) bindIO :: IO a -&gt; (a -&gt; IO b) -&gt; IO b bindIO (IO m) k = IO $ \ s -&gt; case m s of (# new_s, a #) -&gt; unIO (k a) new_s </code></pre> <p>where <code>unIO</code> simply unwraps the function from inside the <code>IO</code> constructor.</p> <p>It's important to note that <code>IO a</code> represents a <em>description</em> of an impure computation that could be run to produce a value of type <code>a</code>. The fact that there's a way to get values out of GHC's internal representation of <code>IO</code> doesn't mean that this holds in general, or that you can do such a thing for all monads. It's purely an implementation detail on the part of GHC.</p> <p><sup>1</sup> The <a href="http://learnyouahaskell.com/for-a-few-monads-more#state">state monad</a> is a monad used for accessing and mutating a state across a series of computations; it's represented as <code>s -&gt; (a, s)</code> (where <code>s</code> is the type of state), which looks very similar to the type GHC uses for <code>IO</code>, thus the confusion.</p>
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    1. POWhat are the definitions for >>= and return for the IO monad?
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    1. POWhat are the definitions for >>= and return for the IO monad?
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    1. CO+1 for the great explanation for why State# is used to preserve statement order!
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      1. USaelguindy
    2. COI disagree that IO is not a State Monad. It is only that the State that it carried around (commonly called `RealWorld`) is completely abstract. This fact no more disqualifies it as a true State Monad then would the abstract nature of the `s`-thread in the `ST` Monad.
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      1. USJohn F. Miller
    3. CO@JohnF.Miller: The state monad model of `IO` can model sequential computation (even though `IO` is not implemented in that way in practice), but it cannot reasonably represent concurrency. Anyway, it's definitely more confusing than helpful, given that such a model of `IO` is *much* more restricted than a state monad (or you'd be able to do things like putting the state of the world back after performing some side-effects, reversing them).
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