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    <p><code>let foo = bar in ...</code> simply defines <code>foo</code> to be the exact same thing as <code>bar</code> within the context of <code>...</code>; you could simply use textual substitution to replace all uses of <code>foo</code> in <code>...</code> with <code>(bar)</code> and get the exact same result.</p> <p><code>where</code> clauses are similar to <code>let...in</code> expressions, but go at the end of a function clause, instead of being an expression. For instance,</p> <pre><code>foo x | p1 = ... y ... | p2 = ... y ... where y = ... </code></pre> <p>There is no way to rewrite this with <code>let...in</code> without changing the guards into <code>if...then...else</code>s. Often, <code>where</code> clauses are used over <code>let...in</code> clauses purely for reasons of style.</p> <p>The bind operator is something different entirely. It's used in <code>do</code> notation to "extract" a value from a monadic computation. That is, if <code>foo</code> has the type <code>m a</code>, then after <code>x &lt;- foo</code>, <code>x</code> has the type <code>a</code>. All the other "binding" forms just define names, but <code>&lt;-</code> is used for <em>binding</em> a computation's result to a name from within a monad. <code>&lt;-</code> can only be used inside a <code>do</code> block, so it's exclusively used to build up a larger computation in the same monad as the action you're binding the result of.</p> <p><code>let foo = bar</code> in <code>do</code> notation is just a convenience; you can rewrite:</p> <pre><code>do let foo = bar ... </code></pre> <p>as</p> <pre><code>let foo = bar in do ... </code></pre> <p>but that gets messy when you have a lot of such bindings, as the <code>do</code> blocks get nested deeper and deeper.</p> <p>I don't know what the advice you mentioned is talking about; you can define multiple variables in a <code>let...in</code> block just fine, and</p> <pre><code>let foo = ... bar ... in ... </code></pre> <p>is more idiomatic than</p> <pre><code>let foo = ... in let bar = ... in ... </code></pre>
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