Question

FilmsSorted :: [Film] -> [Film]  
FilmsSorted = reverse.sortBy sortFilm

我收到此错误，不确定如何修复它？（下同）

******。hs：89：1：错误：无效的类型签名：FilmsSorted ::
应该是::

形式

无效的签名？

Answer 1

请发布完整示例，例如：

import Data.List (reverse, sortBy)
type Film = ()

FilmsSorted :: [Film] -> [Film]
FilmsSorted = reverse . sortBy sortFilm

sortFilm :: Film -> Film -> Bool
sortFilm = undefined

一个完整的，格式化的错误：

GHCi, version 8.4.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/tommd/.ghci
[1 of 1] Compiling Main             ( x.hs, interpreted )

x.hs:4:1: error:
    Invalid type signature: FilmsSorted :: ...
    Should be of form <variable> :: <type>
  |
4 | FilmsSorted :: [Film] -> [Film]
  | ^^^^^^^^^^^
Failed, no modules loaded.

此时错误表明您的变量FilmsSorted在某种程度上是不正确的。正如@Cubic所说，错误是变量必须以小写字母开头。所以试试：

filmsSorted :: ...
filmsSorted  = ...

Haskell'无效类型签名'

1 个答案: