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    <p>There are two techniques that are used by purely functional programming languages to model side effects:</p> <p>1) A world type that represents external state, where each value of that type is guaranteed by the type system to be used only once.</p> <p>In a language that uses this approach the function <code>print</code> and <code>read</code> might have the types <code>(string, world) -&gt; world</code> and <code>world -&gt; (string, world)</code> respectively.</p> <p>They might be used like this:</p> <pre><code>let main w = let w1 = print ("What's your name?", w) in let (str, w2) = read w1 in let w3 = print ("Your name is " ^ name, w2) in w3 </code></pre> <p>But not like this:</p> <pre><code>let main w = let w1 = print ("What's your name?", w) in let (str, w2) = read w in let w3 = print ("Your name is " ^ name, w2) in w3 </code></pre> <p>(because w is used twice)</p> <p>All built-in functions with side-effects would take and return a world value. Since all functions with side-effects are either built-ins or call other functions with side-effects, this means that all functions with side-effects need to take and return a world.</p> <p>This way it is not possible to call a function with side-effects twice with the same arguments and referential transparency is not violated.</p> <p>2) An IO monad where all operations with side effects have to be executed inside that monad.</p> <p>With this approach all operations with side effects would have type <code>io something</code>. For example <code>print</code> would be a function with type <code>string -&gt; io unit</code> and <code>read</code> would have type <code>io string</code>.</p> <p>The only way to access the value of performing operation would be to use the "monadic bind" operation (called >>= in haskell for example) with the IO operation as one argument and a function describing what to do with the result as the other operand.</p> <p>The example from above would look like this with monadic IO:</p> <pre><code>let main = (print "What's your name?") &gt;&gt;= (lambda () -&gt; read &gt;&gt;= (lambda name -&gt; print ("Your name is " ^ name))) </code></pre>
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    1. POHow do functional languages model side-effects?
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    1. POHow do functional languages model side-effects?
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    1. COYour #2 is an implementation detail for your #1. The IO monad would just automatically carry around that world state for you, packaging and unpackaging it as needed behind whatever syntax your monad uses.
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    2. CO@JUST: You mean #1 is an implementation detail of #2, no? But then it's not an implementation detail if it's the interface that's exposed to the programmer.
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    3. COYeah, right. Sorry. #1 is an implementation detail of #2.
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