November 2017
Beginner to intermediate
344 pages
7h 2m
Chinese
Content preview from 深入理解運算原理｜從簡單的機器到無所不能的程式
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226
|
第 7 章
利用
#minimize
，並且重複呼叫用來執行單一模擬步驟的基元遞迴函式，就可以完全模
擬圖靈機。模擬物會持續到機器停止，但如果一直不發生，就會永遠執行。
SKI 組合器演算
SKI
組合器演算
（
SKI combinator calculus
）是操縱運算式語法的規則系統，就如同 lambda
演算。雖然 lambda 演算已經非常簡單，但它還是有 3 種運算式（變數、函式、呼叫），
而我們曾在第 208 頁『語意』看到，變數會讓化簡的規則變得有點複雜。SKI 演算甚至
更為簡單，只有兩種運算式呼叫和字母
符號
（
symbol
），而且規則也更為簡單。它所有
的威力來自 3 個特殊符號：
S
、
K
、
I
，稱為
組合器
（
combinator
），每個都有自己的化簡
規則：
• 將
S[
a
][
b
][
c
]
化簡成
a
[
c
][
b
[
c
]]
，其中
a
、
b
、
c
可以是任何的 SKI 演算運算式。
• 將
K[
a
][
b
]
化簡成
a
。
• 將
I[
a
]
化簡成
a
。
例如以下是化簡運算式
I[S][K][S][I[K]]
的其中一種方式：
I[S][K][S][I[K]] → S[K][S][I[K]] (
reduce I[S] to S
)
→ S[K][S][K] (
reduce I[K] to K
)
→ K[K][S[K]] (
reduce S[K][S][K] to K[K][S[K]]
)
→ K (
reduce K[K][S[K]] to K
)
請注意，這裡沒有繼續替換 lambda 演算樣式的變數，只是根據化簡規則針對符號進行 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.