test n = (sum s)*4+2*(n-1)*((l')^2)-2*(n-1)*(l')
where
p =sort $[ m | a<-[1..(n-1)],b<-[1..(n-1)],let m= (b/a), (a^2+b^2< (n^2))]
l'= length p
s = (product a) : next a (group p)
where
a = [(n-1),2*(l')+(n-2)]
next [x,y] (z:z':zs) = case (null zs) of
False -> (x')*(y')*l : next [x',y'] (z':zs)
True -> (x')*(y')*l :[]
where
l = length z
x' = x+l
y' = y-length z'
爲什麼上面的代碼提供以下錯誤:替代n的[X | X < - [1..N]]
No instance for (Fractional Int) arising from a use of ‘/’ In the expression: (b/a)
In an equation for ‘m’: m = (b/a)
In the second argument of ‘($)’, namely
‘[m | a <- [1 .. (n - 1)], b <- [1 .. (n - 1)], let m = (b/a), (a^2 + b^2 < (n^2))]’
但是,當我在代碼取代n=3
手工:
test' = (sum s)*4+2*(3-1)*((l')^2)-2*(3-1)*(l')
where
p = sort $[ m | a<-[1..(3-1)],b<-[1..(3-1)],let m= (b/a), (a^2+b^2< (3^2))]
l'= length p
s = (product a) : next a (group p)
where
a = [(3-1),2*(l')+(3-2)]
next [x,y] (z:z':zs) = case (null zs) of
False -> (x')*(y')*l : next [x',y'] (z':zs)
True -> (x')*(y')*l :[]
where
l = length z
x' = x+l
y' = y-length z'
然後它可以在GHCi中運行;發生什麼事?
您可能會發現有趣的是,知道類型理論家通常會證明他們的系統是beta測試重視打字;也就是說,如果'(\ x - > e)e''可以被賦予't'類型,那麼'e [e'/ x]'(我們用'e'替換' e')也可以被賦予't'類型。然而,相反的情況通常並不正確:'e [e'/ x]'可能比'(\ x - > e)e''更多的類型。特別是當'e''是多態的時候 - 因爲'3'在你的例子中 - 但被函數應用強制爲單態。 –