为什么这个不能加载到GHCI中

Question

let partialsums lst = reverse ( partial_sums lst 0 [] ) where
                      partial_sums ls accum accumulator_list = 
                      if ls == [] then accumulator_list else
                      partial_sums ( tail ls ) ( accum + head ls ) 
                      ((accum + head ls) : accumulator_list)

谢谢你的帮助。Haskell看起来很有趣，但如果没有一个好教授的指导和指导，这是一门很难学的语言。我只是想自学这门语言。

Answer 1

这不是空格--而是let（正如chi提到的，您必须缩进partial_sums的正文--对不起，我一开始没有检查这个）：

如果要在.hs文件中定义函数，则应编写如下内容：

partialsums lst = reverse ( partial_sums lst 0 [] ) where
                  partial_sums ls accum accumulator_list = 
                     if ls == [] then accumulator_list else
                     partial_sums ( tail ls ) ( accum + head ls ) 
                     ((accum + head ls) : accumulator_list)

虽然我会写得更像这样：

partialsums lst = reverse ( partial_sums lst 0 [] ) 
   where
      partial_sums ls accum accumulator_list = 
         if ls == [] 
         then accumulator_list 
         else
            partial_sums ( tail ls ) ( accum + head ls ) 
                  ((accum + head ls) : accumulator_list)

partialsums :: Num a => [a] -> [a]
partialsums lst = reverse ( partial_sums lst 0 [] ) 
   where
      partial_sums [] _ accumulator_list = accumulator_list
      partial_sums (h:tl) accum accumulator_list =
            let accum' = accum + h in
            partial_sums tl accum' (accum' : accumulator_list)

> partialsums [1..5]
[1,3,6,10,15]

partialSums :: Num a => [a] -> [a]
partialSums = scanl1 (+)