Haskell代码中的编译器错误

Question

我正在尝试使用where子句编写椭圆曲线点加法.我收到编译器错误但是当我使用let in expression翻译相同的代码时,它工作正常.有人可以告诉我这段代码有什么问题.完整源代码[ http://hpaste.org/49174 ]
谢谢
Mukesh Tiwari

{--
--add points of elliptic curve using where clause getting compiler error
addPoints :: Elliptic -> Point -> Point -> Either Point Integer
addPoints _ Identity p_2 = Left p_2
addPoints _ p_1 Identity = Left p_1
addPoints ( Conelliptic a b n ) ( Conpoint x_p y_p ) ( Conpoint x_q y_q )  
    | x_p /= x_q =  case ( ( Conpoint x_r y_r ) , d ) of
                        ( _ , 1 ) -> Left ( Conpoint x_r y_r )
                        ( _ , d' ) -> Right d'
                      where
                                [ u , v , d ] = extended_gcd ( x_p - x_q ) n
                                s = mod ( ( y_p - y_q ) * u ) n
                                x_r = mod ( s*s - x_p - x_q ) n
                                y_r = mod ( -y_p - s * ( x_r - x_p ) ) n
    | otherwise =   if mod ( y_p + y_q ) n == 0 then Left Identity
                     else  case ( ( Conpoint x_r y_r ) , d ) of
                                ( _ , 1 ) -> Left ( Conpoint x_r y_r )
                                ( _ , d' ) -> Right d'
                            where
                                [ u , v , d ] = extended_gcd ( 2 * y_p ) n
                                s = mod ( ( 3 * x_p * x_p + a ) * u ) n
                                x_r = mod ( s * s - 2 * x_p ) n
                                y_r = mod ( -y_p - s * ( x_r - x_p ) ) n 

--}


--add points of elliptic curve let in clause and its working
addPoints::Elliptic->Point->Point-> Either Point Integer
addPoints _ Identity p_2 = Left p_2
addPoints _ p_1 Identity = Left p_1
addPoints ( Conelliptic a b n ) ( Conpoint x_p y_p ) ( Conpoint x_q y_q )
| x_p /= x_q = let
           [ u , v , d ] = extended_gcd (x_p-x_q) n
           s = mod  ( ( y_p - y_q ) * u ) n
           x_r = mod ( s * s - x_p - x_q ) n
           y_r= mod ( -y_p - s * ( x_r - x_p ) ) n
         in case ( ( Conpoint x_r y_r ) , d ) of
          ( _ , 1 ) -> Left ( Conpoint x_r y_r )
          ( _ , d' ) -> Right d'
| otherwise = if mod ( y_p + y_q ) n == 0 then Left Identity
         else  let
              [ u , v , d ] = extended_gcd ( 2*y_p ) n
              s = mod  ( ( 3 * x_p * x_p + a ) * u ) n
              x_r = mod ( s * s - 2 * x_p ) n
              y_r = mod ( -y_p - s * ( x_r - x_p ) ) n
           in case ( ( Conpoint x_r y_r ) , d ) of
                            ( _ , 1 )-> Left (Conpoint x_r y_r)
                            ( _ , d' ) -> Right d'

Answer 1

问题是where阻止功能保护的范围,因此不可能where为每个保护语句创建单独的.当ghc遇到第where59行时,它会自动结束函数声明并期望跟随一个新的声明,这会|产生错误,因为它不是一个有效的声明.它与let-expression一起使用,因为let它where是语言的不同部分.该哈斯克尔维基对这个主题的更多信息.