Connection To Intuitionistic Logic
The combinators K and S correspond to two well-known axioms of sentential logic:
AK: A (B A),
AS: (A (B C)) ((A B) (A C)).
Function application corresponds to the rule modus ponens:
MP: from A and A B, infer B.
The axioms AK and AS, and the rule MP are complete for the implicational fragment of intuitionistic logic. In order for combinatory logic to have as a model:
- The implicational fragment of classical logic, would require the combinatory analog to the law of excluded middle, e.g., Peirce's law;
- Complete classical logic, would require the combinatory analog to the sentential axiom F A.
Read more about this topic: SKI Combinator Calculus
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