小编mag*_*mig的帖子

我开始用haskell编程.我正在开发的程序只是将一个列表的总和与两个元素相加,例如:

[("book",10),("cookies",2),("icecream",5)]

这应该返回"17".这是我的代码:

total [] = []
total ([("c",e)]:y) = total y ++ [e]

但是在GHCi中运行它会给我这个错误:

<interactive>:80:8:
    Couldn't match expected type `[([Char], a0)]'
                with actual type `([Char], t0)'
    In the expression: ("livro", 10)
    In the first argument of `total', namely
      `[("livro", 10), ("bolachas", 2), ("gelado", 5)]'
    In the expression:
      total [("livro", 10), ("bolachas", 2), ("gelado", 5)]

<interactive>:80:21:
    Couldn't match expected type `[([Char], a0)]'
                with actual type `([Char], t1)'
    In the expression: ("bolachas", 2)
    In the first argument of `total', namely
      `[("livro", …

我的课程是通过这样的输入来总结9级以上(从0到20)的学生数量:

aprov [("John",14)("Martha",8)("Elsa",12)]

输出应为"2".但在编译时:

k = 0
aprov [] = 0
aprov ((a,n):y)  = if n > 9 then k == k + 1 else k == k

GHCi给了我这个错误:

Could not deduce (Num Bool) arising from the literal `0'
from the context (Num a, Ord a)
  bound by the inferred type of
     aprov :: (Num a, Ord a) => [(t, a)] -> Bool
  at prog.hs:(29,1)-(30,55)
Possible fix: add an instance declaration for (Num Bool)
In the expression: 0
In an …