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copied!<p>Sadly the identity monad needs at least a newtype wrapper to dispatch upon.</p> <p>The reason makes sense when you consider overlapping instances. A monad instance that was somehow constructed for </p> <pre><code>type Id a = a </code></pre> <p>would always apply, subsuming any of the more interesting monads.</p> <p>Therefore you have to implement it as something like</p> <pre><code>newtype Id a = Id a </code></pre> <p>Then</p> <pre><code>instance Monad Id where return = Id Id a >>= f = f a </code></pre> <p>This gives a sufficient hint to the type system to know when you really want to work with this monad. Note: This is an artifact of typeclass dispatch. In languages that offer ML-like modules or Scala, where you don't have typeclass inference to help (and conversely to worry about) you can define a true identity monad, but then you are forced to route all calls to that particular >>= by yourself, so it turn to be a wash or a net loss anyways.</p> <p>Now you can</p> <pre><code>do x <- computeSomething y <- computeSomethingElse doStuff x y </code></pre> <p>but the types are</p> <pre><code>computeSomething :: Id a computeSomethingElse :: Id b doStuff :: a -> b -> Id c </code></pre> <p>and if you made Id into an instance of Applicative the syntax could reduce to</p> <pre><code>doStuff <$> computeSoemthing <*> computeSomethingElse </code></pre> <p>so each of those methods would have to explicitly apply the Id newtype wrapper or wrap their result in return.</p> <p>In practice it is better to use let bindings in this context.</p> <pre><code>let x = computeSomething y = computeSomethingElse in doStuff x y </code></pre> <p>or more idiomatically, a where clause</p> <pre><code>doStuff x y where x = computeSomething y = computeSomethingElse </code></pre> <p>or even in this case, since those are so small, and to come full circle to the Applicative example above:</p> <pre><code>doStuff computeSomething computeSomethingElse </code></pre>

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