试图从生成的列表总结

Question

你好，我最近开始学习Haskell和我试图做的是给它创建了一个功能列表的小程序，然后我想利用列表的总和：

f a b c = a+b+c 

my_sum [] = 0 
my_sum (x:xs) = x + my_sum xs 

my_list f a b c = [f a b x |x <- [1..c]] 

我试图采取列表的总和是这样，但你能帮助我，我总是得到错误

*Main> my_sum [my_list f 1 1 4] 

<interactive>:13:1: 
    No instance for (Num [t0]) arising from a use of `my_sum' 
    Possible fix: add an instance declaration for (Num [t0]) 
    In the expression: my_sum [my_list f 1 1 4] 
    In an equation for `it': it = my_sum [my_list f 1 1 4] 

<interactive>:13:9: 
    No instance for (Num t0) arising from a use of `my_list' 
    The type variable `t0' is ambiguous 
    Possible fix: add a type signature that fixes these type variabl 
    Note: there are several potential instances: 
     instance Num Double -- Defined in `GHC.Float' 
     instance Num Float -- Defined in `GHC.Float' 
     instance Integral a => Num (GHC.Real.Ratio a) 
     -- Defined in `GHC.Real' 
     ...plus three others 
    In the expression: my_list f 1 1 4 
    In the first argument of `my_sum', namely `[my_list f 1 1 4]' 
    In the expression: my_sum [my_list f 1 1 4] 

？

Answer 1

my_sum需要一个参数，一个数字列表。由于my_list返回一个列表，其包裹在结果列表结果列表的列表（不匹配my_sum）：

my_sum [my_list f 1 1 5] -- argument has type Num a => [[a]] 
my_sum (my_list f 1 1 5) -- this is right