The kata is inspired by a Stack Overflow question.
It is easy to prove that f(f(f b)) = f b for all functions f : bool -> bool. But can you do it in less than 92 characters?
More specifically, your task is to prove the following lemma:
Lemma lemma : forall (f : bool -> bool) (b : bool), f (f (f b)) = f b.
And the size of your solution (including all declarations) should be 91 characters or less.
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Top comments (7)
Here's my solution in Coq:
The second line has 80 characters.
Not a code golfer, but pretty compact with writing the way I learned in uni:
Above code is 82 characters, excluding whitespace.
EDIT: now in LaTeX
Another proof in Isabelle/HOL
In Isabelle/HOL
You have a typo in the lemma statement, f is applied twice instead of three times.
In Isabelle/HOL equivalent to the above but with a different notation
And with another notation