Question

我对Haskell很新，我想解决一些问题。我正在尝试创建一个小程序，它将从text.txt文件中读取表达式并返回结果。

首先，我阅读了文本并将我读到的内容转换为字符串：

module Main where
import Data.Char

convert :: String -> String
convert = unlines . (map convertLine) . lines

convertLine :: String -> String
convertLine = unwords . (map convertWord) . words

convertWord :: String -> String
convertWord s = (toUpper (head s)):(tail s)

main = do
    name <- readFile "test.txt"
    putStr $ convert name

现在，我想解析该字符串，我从：

开始

parse :: (Read a, Num a, Fractional a) => String -> [SyntacticalElement a]
parse "" = []
parse putStr = element : (parse rest)
                   where (element, rest) = next_elem putStr

我在这里得到错误：next_elem不在范围内知道为什么吗？

更新

module Main where
import Data.Char

convert :: String -> String
convert = unlines . (map convertLine) . lines

convertLine :: String -> String
convertLine = unwords . (map convertWord) . words

convertWord :: String -> String
convertWord s = (toUpper (head s)):(tail s)

main = do
    name <- readFile "test.txt"
    putStr $ convert name

number = ['0'..'9']  ++ ['.']
operator = ['+', '-', '*', '/']
open_brackets  = ['(', '[']
close_brackets = [')', ']']
brackets = open_brackets ++ close_brackets
allowed_chars = number ++ operator ++ brackets

parse :: (Read a, Num a, Fractional a) => String -> [SyntacticalElement a]
parse "" = []
parse putStr = element : (parse rest)
                   where (element, rest) = next_elem putStr
next_elem :: (Read a, Num a, Fractional a) => String -> (SyntacticalElement a, String)
next_elem s@(first:_)
| is_open_bracket first  = (to_sublist content, rest_b)
| is_operator first      = (to_operator operator, rest_o)
| is_number first        = (to_number number, rest_n)
| is_close_bracket first = error "Unexpected closing bracket!"
| otherwise              = error $ "Invalid Expression: \"" ++ s ++ "\""
where (number,   rest_n) = span is_number s
      (operator, rest_o) = span is_operator s
      (content,  rest_b) = parse_bracket s

Answer 1

因为编译器不知道next_elem的含义。为什么会这样？在哪里定义？

对于这些情况，使用interact通常很方便，而不是费心去读取文件。

Haskell - 从文件读取，解析表达式，计算和返回结果

1 个答案: