用opt解析式解析"enum“选项

Question

如何从grep --help实现此示例的解析器

 --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

我如何编写解析器呢？option auto似乎是一个传说，因为Read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'."