haskell - 为什么我在输入 `<-' 上收到“解析错误”？

Question

这是我的代码

lotto blacklista size board current = [black | blacklista<-action current , warunki blacklista,
                                                if end current
                                                   then if check board blacklista
                                                   then black <- blacklista
                                                          else False
                                                else black <- lotto blacklista size board (next current) ]

你能告诉我为什么我在这一行得到解析错误吗？

then black <- blacklista

Answer 1

从您提供的任何上下文来看，您似乎想要这样的东西：

lotto blacklista size board current = let blist = action current warunki blacklista in
                                          [black | black <- if end current && check board blist then blist
                                                            else lotto blacklista size board (next current)] 

Answer 2

您可以在and之后尝试 Monad（新do块）。thenelse