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    <p>I agree with camccann's answer that it's probably missing mainly because it would have to be baked quite deeply into the implementation and it is of too little use for this level of complication. In Erlang (and Prolog and Lisp) symbols (or atoms) usually serve as special markers and serve mostly the same notion as a constructor. In Lisp, the dynamic environment includes the compiler, so it's partly also a (useful) compiler concept leaking into the runtime.</p> <p>The problem is the following, symbol interning is impure (it modifies the symbol table). Because we never modify an existing object it is referentially transparent, however, but if implemented naïvely can lead to space leaks in the runtime. In fact, as currently implemented in Erlang you can actually crash the VM by interning too many symbols/atoms (current limit is 2^20, I think), because they can never get garbage collected. It's also difficult to implement in a concurrent setting without a huge lock around the symbol table.</p> <p>Both problems can be (and have been) solved, however. For example, see <a href="http://www.erlang.org/eeps/eep-0020.html" rel="noreferrer">Erlang EEP 20</a>. I use this technique in the <a href="http://hackage.haskell.org/package/simple-atom" rel="noreferrer">simple-atom</a> package. It uses <code>unsafePerformIO</code> under the hood, but only in (hopefully) rare cases. It could still use some help from the GC to perform an optimisation similar to indirection shortening. It also uses quite a few <code>IORef</code>s internally which isn't too great for performance and memory usage.</p> <p>In summary, it can be done but implementing it properly is non-trivial. Compiler writers always weigh the power of a feature against its implementation and maintenance efforts, and it seems like first-class symbols lose out on this one.</p>
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    1. POWhy doesn't Haskell have symbols (a la ruby) / atoms (a la erlang)?
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    1. CO+1 for providing more information on the cost of implementing atoms. I am beginning to agree that atoms are perhaps not terribly useful in Haskell.
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