haskell - 使用 optparse-applicative 解析 "enum"选项

Question

我如何为来自 grep --help 的示例实现解析器:

 --binary-files=TYPE   assume that binary files are TYPE;
                       TYPE is 'binary', 'text', or 'without-match'

假设我有

data BinaryFiles = Binary | Text | WithoutMatch

我如何编写解析器？ option auto自 Read 以来似乎是一团糟应该是 Show 的“逆” ，我想保留派生的 instance Show BinaryFiles .

Answer 1

使用 str而不是 auto :

binFile :: ReadM BinaryFiles
binFile = str >>= \s -> case s of
    "binary"        -> return Binary
    "text"          -> return Text
    "without-match" -> return WithoutMatch
    _ -> readerError "Accepted binary file types are 'binary', 'text', and 'without-match'."