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    <p>It's useful as an argument to <a href="http://www.haskell.org/haskellwiki/Higher_order_function#Definition" rel="noreferrer">higher order functions</a> (functions which take functions as arguments), where you want some particular value left unchanged.</p> <p><strong>Example 1</strong>: Leave a value alone if it is in a Just, otherwise, return a default of 7.</p> <pre><code>Prelude Data.Maybe&gt; :t maybe maybe :: b -&gt; (a -&gt; b) -&gt; Maybe a -&gt; b Prelude Data.Maybe&gt; maybe 7 id (Just 2) 2 </code></pre> <p><strong>Example 2</strong>: building up a function via a fold:</p> <pre><code>Prelude Data.Maybe&gt; :t foldr (.) id [(+2), (*7)] :: (Num a) =&gt; a -&gt; a Prelude Data.Maybe&gt; let f = foldr (.) id [(+2), (*7)] Prelude Data.Maybe&gt; f 7 51 </code></pre> <p>We built a new function <code>f</code> by folding a list of functions together with <code>(.)</code>, using <code>id</code> as the base case.</p> <p><strong>Example 3</strong>: the base case for functions as monoids (simplified).</p> <pre><code>instance Monoid (a -&gt; a) where mempty = id f `mappend` g = (f . g) </code></pre> <p>Similar to our example with fold, functions can be treated as concatenable values, with <code>id</code> serving for the empty case, and <code>(.)</code> as append.</p> <p><strong>Example 4</strong>: a trivial hash function.</p> <pre><code>Data.HashTable&gt; h &lt;- new (==) id :: IO (HashTable Data.Int.Int32 Int) Data.HashTable&gt; insert h 7 2 Data.HashTable&gt; Data.HashTable.lookup h 7 Just 2 </code></pre> <p>Hashtables require a hashing function. But what if your key is already hashed? Then pass the id function, to fill in as your hashing method, with zero performance overhead.</p>
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    1. USDon Stewart
    1. USDon Stewart
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    1. COWould it be better to use `foldl' (.) id` over `foldr (.) id`?
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      1. USThomas Eding
    2. CONot in this case. Strictness won't have any effect on function composition.
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    3. CO@Don Stewart, I hope compiler doing something terrible wrong with some functions (like `(+)` and other foreign-functions which requires both arguments to be bound) like changing tree of application to fold some nodes and free memory used by them.
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