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I recently posted a code-golf answer in Scheme-1. However, the Scheme-1 language cannot be implemented (as it would require the implementation to solve the halting problem).

I marked it as non-competing because there is no implementation. However, @ais523 commented:

I'm not sure that you can submit an answer in a language that's theoretically impossible to implement (assuming the Church-Turing hypothesis), because we normally go by the implementation to define how the language works, and that doesn't work in this case. Non-competing status is normally reserved for languages where there's a viable implementation, but it's newer than the challenge.

What is our policy on uncomputable submissions?

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    \$\begingroup\$ The implementation defines the language. Unimplemented languages do not exist as far as PPCG is concerned. \$\endgroup\$ – Dennis Apr 25 '17 at 2:11
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    \$\begingroup\$ Feel free to post it as non-competing - we're pretty permissive when it comes to non-competing answers. \$\endgroup\$ – isaacg Apr 25 '17 at 15:40
  • \$\begingroup\$ @Dennis Sounds like an answer to this. \$\endgroup\$ – Adám Jul 2 at 4:50
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If there is no implementation, there is no language. We require a functioning implementation of the language (as a compiler, interpreter, or whatever) for it to be used on PPCG. Since Scheme-1's implementation requires solving the halting problem, and the halting problem cannot be solved, Scheme-1 cannot be used on PPCG.

More generally: if there is no implementation, there is no language, and any answer written in that "language" would be invalid and subject to removal per our policy on invalid answers.

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    \$\begingroup\$ So I can't post it whatsoever? Not even if I mark it as invalid in some way? \$\endgroup\$ – Esolanging Fruit Apr 25 '17 at 5:02
  • \$\begingroup\$ @Challenger5 Nope. See our policy on invalid answers. \$\endgroup\$ – Mego Apr 25 '17 at 5:03
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    \$\begingroup\$ "if the only reason the answer is invalid is because there was no acceptable/legal interpreter for the language at the time the question was written, an answer may be submitted as non-competing." That applies to Scheme-1. \$\endgroup\$ – Esolanging Fruit Apr 25 '17 at 5:09
  • \$\begingroup\$ @Challenger5 Yes, but that implies that, at the time of the answer's posting, there is an acceptable/legal interpreter. Because there can never be an interpreter, it's invalid and thus not acceptable. \$\endgroup\$ – Mego Apr 25 '17 at 5:16
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    \$\begingroup\$ What about languages for which an interpreter will be written in the future? \$\endgroup\$ – Esolanging Fruit Apr 25 '17 at 6:17
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    \$\begingroup\$ @Challenger5 Let me try to be more clear: if there is no interpreter, the answer is not valid. An interpreter must exist prior to posting the answer for it to be valid. \$\endgroup\$ – Mego Apr 25 '17 at 6:18
  • \$\begingroup\$ Sounds like an answer to this. \$\endgroup\$ – Adám Jul 2 at 4:50

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