防止外部编辑Application Manifest？(Prevent external editing of an Application Manifest?)

 函数eval返回一个Int ，但是在evExpr您试图将该Int推入一个Maybe Integer （基于evExpr的类型签名）。 Int不是Integer ，所以你得到一个错误。 您可以将eval更改为返回Integer ，将evExpr更改为返回Maybe Int ，或将Int转换为fromIntegral的Integer 。 
 
 第二个错误发生是因为runParser有一个类型签名： 
 
runParser :: GenParser tok st a -> st -> SourceName -> [tok] -> Either ParseError a
 
 但是你用一个Parser Expr和一个String来调用它，并且忽略了SourceName和st参数。 
 
 我猜你的解析器没有任何用户状态，所以你可以使用： 
 
parse :: Stream s Identity t => Parsec s () a -> SourceName -> s -> Either ParseError a
 
 这不需要用户状态（因为它具有type () ），但仍需要为源传递名称。 就像是： 
 
run :: String -> Either ParseError Expr
run = parse expr "<string>"
 
 你将遇到的下一个问题是，如果解析器遇到解析错误，那么run无法返回Expr 。 解析器返回一个Either ParserError Expr ，你必须在evExpr处理Either ParserError Expr 。 实际上，在evExpr中检查空的列表/ String是没有evExpr ，因为解析器会处理它（并生成一个错误）。 你最好将evExpr更改为返回一个Either ParseError Int ，并且仅对解析器结果的eval进行fmap 。 
 
evExpr :: String -> Either ParseError Int
evExpr str = eval <$> run str

The function eval returns an Int, but in evExpr you are trying to shove that Int into a Maybe Integer (based upon the type signature of evExpr). An Int is not an Integer, so you get an error. You can either change eval to return an Integer, change evExpr to return a Maybe Int, or convert the Int into an Integer with fromIntegral.
 
The second error is occurring because runParser has a type signature:
 
runParser :: GenParser tok st a -> st -> SourceName -> [tok] -> Either ParseError a
 
But you are calling it with a Parser Expr and a String, and left out the SourceName and st parameters.
 
I'm guessing that your parser doesn't have any user state, so you could use:
 
parse :: Stream s Identity t => Parsec s () a -> SourceName -> s -> Either ParseError a
 
Which doesn't require the user state (since it has type ()), but still requires you to pass a name for the source. Something like:
 
run :: String -> Either ParseError Expr
run = parse expr "<string>"
 
The next problem you're going to run into is that run can't return an Expr if the parser encounters a parse error. The parser returns an Either ParserError Expr, which you'll have to deal with in evExpr. In fact, there's no point in checking for an empty list/String in evExpr, since the parser will handle it (and generate an error). You're better off changing evExpr to return an Either ParseError Int, and just fmap the eval over the parser result.
 
evExpr :: String -> Either ParseError Int
evExpr str = eval <$> run str