问题
I'm having trouble converting the lambda for flip into the SKI combinators (I hope that makes sense). Here is my conversion:
/fxy.fyx
/f./x./y.fyx
/f./x.S (/y.fy) (/y.x)
/f./x.S f (/y.x)
/f./x.S f (K x)
/f.S (/x.S f) (/x.K x)
/f.S (/x.S f) K
/f.S (S (/x.S) (/x.f)) K
/f.S (S (K S) (K f)) K
S (/f.S (S (K S) (K f))) (/f.K)
S (/f.S (S (K S) (K f))) (K K)
S (S (/f.S) (/f.S (K S) (K f))) (K K)
S (S (K S) (/f.S (K S) (K f))) (K K)
S (S (K S) (S (/f.S (K S)) (/f.K f))) (K K)
S (S (K S) (S (/f.S (K S)) K)) (K K)
S (S (K S) (S (S (/f.S) (/f.K S)) K)) (K K)
S (S (K S) (S (S (K S) (/f.K S)) K)) (K K)
S (S (K S) (S (S (K S) (S (/f.K) (/f.S))) K)) (K K)
S (S (K S) (S (S (K S) (S (K K) (K S))) K)) (K K)
If I understand properly in the B, C, K, W system, C is flip and it's definition in SKI terms is S (S (K (S (K S) K)) S) (K K). obviously my answer isn't the same as the C combinator... Here are the rules I used for the conversions:
K y = /x.y - K combinator
(SKK) = /x.x - I combinator
(S (/x.f) (/x.g)) = (/x.fg) - S combinator
y = (/x.yx) - eta reduction
/x./y.f = /xy.f - currying
Note that the S rule can convert longer expressions - for example, λ x.abcdeg can be converted by setting f = abcde.
What am I missing?
回答1:
Once the answer being accepted I revised it and I figured out it is actually wrong.
Your final result is correct, although it is not the "official" answer in the text book, but it is possible that different SKI terms are actually equivalent to the same lambda expression.
S [S (K S) (S (S (K S) (S (K K) (K S))) K)] [K K] f x y
-> S (K S) (S (S (K S) (S (K K) (K S))) K) f (K K f) x y
-> K S f (S (S (K S) (S (K K) (K S))) K f) (K K f) x y
-> S [S (S (K S) (S (K K) (K S))) K f] (K K f) x y
-> S [S (K S) (S (K K) (K S))] K f x (K K f x) y
-> S [K S] [S (K K) (K S)] f (K f) x (K K f x) y
-> K S f (S (K K) (K S) f) (K f) x (K K f x) y
-> S [S (K K) (K S) f] [K f] x (K K f x) y
-> S [K K] [K S] f x (K f x) (K K f x) y
-> K K f (K S f) x (K f x) (K K f x) y
-> K (K S f) x (K f x) (K K f x) y
-> K S f (K f x) (K K f x) y
-> S [K f x] [K K f x] y
-> K f x y (K K f x y)
-> f y (K K f x y)
-> f y (K x y)
-> f y x
The above derive, based on left most reduction order, proves that your final term is equivalent to the C combinator.
来源：https://stackoverflow.com/questions/29379277/convert-flip-lambda-into-ski-terms