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copied!<p>I personally prefer the <code>/:</code> and <code>:\</code> forms of <code>foldLeft</code> and <code>foldRight</code>. Two reasons:</p> <ol> <li><p>It has a more natural feel because you can see that you are pushing a value into the left/right of a collection and applying a function. That is</p> <pre><code>(1 /: ints) { _ + _ } ints.foldLeft(1) { _ + _ } </code></pre> <p>Are both equivalent, but I tend to think the former emphasises my intuition as to what is happening. If you want to know <em>how</em> this is happening (i.e. the method appears to be called on the value 1, not the collection), it's because methods ending in a colon are right-associative. This can be seen in <code>::</code>, <code>+:</code> etc etc elsewhere in the standard library. </p></li> <li><p>The ordering of the <code>Function2</code> parameters is the same order as the folded element and that which is folded into:</p> <pre><code> (b /: as) { (bb, a) => f(bb, a) } // ^ ^ ^ ^ // ^ ^ ^ ^ // B A B A </code></pre> <p>Better in every way than:</p> <pre><code>as.foldLeft(b) { (bb, a) => f(bb, a) } </code></pre> <p>Although I admit that this was a far more important difference in the era before decent IDE support: nowadays IDEA can tell me what function is expected with a simple CTRL-P</p></li> </ol> <p>I hope it should also be obvious how <code>:\</code> works with foldRight - it's basically exactly the same, except that the value appears to be being pushed in from the right hand side. I must say, I tend to steer well clear of <code>foldRight</code> in scala because of how it is implemented (i.e. wrongly).</p>

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