javascript中的Y-Combinator阶乘适用于数字,而不是教堂数字。
我在javascript中使用ES6 arrow函数实现了Church编码和Y-Combinator。但当我试图计算阶乘函数时javascript中的Y-Combinator阶乘适用于数字,而不是教堂数字。,javascript,lambda-calculus,y-combinator,church-encoding,Javascript,Lambda Calculus,Y Combinator,Church Encoding,我在javascript中使用ES6 arrow函数实现了Church编码和Y-Combinator。但当我试图计算阶乘函数时 FALSE = a => b => b TRUE = a => b => a ZERO = f => z => z ONE = f => z => f(z) SIX = f => z => f(f(f(f(f(f(z)))))) isZERO = n => n(x => FALSE)(TRUE
FALSE = a => b => b
TRUE = a => b => a
ZERO = f => z => z
ONE = f => z => f(z)
SIX = f => z => f(f(f(f(f(f(z))))))
isZERO = n => n(x => FALSE)(TRUE)
SUCC = n => f => z => f(n(f)(z))
MULT = n => m => f => z => n(m(f))(z)
PAIR = a => b => z => z(a)(b)
FIRST = p => p(a => b => a)
SECOND = p => p(a => b => b)
ZZ = PAIR(ZERO)(ZERO)
SS = p => PAIR(SECOND(p))(SUCC(SECOND(p)))
PRED = n => FIRST(n(SS)(ZZ))
FactGen = fact => n =>
isZERO(n)
(ONE)
(MULT(n)(fact(PRED(n))))
Y = g => (x => g(y => x(x)(y))) (x => g(y => x(x)(y)))
Y(FactGen)(SIX) (x=>x+1)(0)
我得到了“UncaughtRangeError:超过最大调用堆栈大小(…)”错误
如果我换了FactGen
FactGen = fact => n => n == 0 ? 1 : n * fact(n - 1)
Y(FactGen)(6)
720
它只是工作
我想知道的是它的教会数字版本。
我怎样才能做到这一点呢?您的问题是JavaScript不是惰性的。具体地说,
isZero
“if”在检查第一个参数是否为零之前,会计算其所有参数
我们可以使用带有单位函数的if
来解决这个问题:
// type Bool = a -> a -> a
// type Lazy a = () -> a
// IF :: Bool -> Lazy a -> Lazy a -> a
IF = c => a => b => c(a)(b)()
FactGen = fact => n =>
IF(isZERO(n))
(()=>ONE)
(()=>MULT(n)(fact(PRED(n))))
// ^^^^
或者省略IF
包装器,将布尔编码直接更改为
// type Bool = Lazy a -> Lazy a -> a
FALSE = a => b => b()
TRUE = a => b => a()
谢谢。贝吉。FactGen=fact=>n=>isZERO(n)(()=>ONE)(()=>MULT(n)(fact(PRED(n)))成功了!