当你有胖箭头的时候，类型类有点乱。类型类Readable的目的是定义可以从给定的ImageFormat读取的类型集，其中ImageFormat也是一个类型类。有点冗长，但下面会很清楚;请原谅我。

class ImageFormat format => Readable img format where
  decode :: format -> B.ByteString -> Either String img
--          |         |               |- returns codification error or the something of type img.
--          |         |- This is the raw bytestring of the image
--          |- This is a type which implements the ImageFormat typeclass  (see below)

现在，您可以在文档中检查每个类型类的示例。例如ImageFormat就有这些示例。

你的第一个论点应该是其中之一。不仅如此，您还需要确保img format对实现了Readable类型类。例如，Readable有以下示例：

• Readable (Image VS RGB Double) PNG
• Readable [Image VS RGB Double] (Seq GIF)

你可以将这些解释为“从一个PNG我可以读和Image VS RGB Double和从一个序列的GIF我可以读[Image VS RGB Double]（图像列表）”。但更重要的是没有（！！）示例

• Readable [Image VS RGB Double] PNG

这意味着你不能从PNG中读取图像列表。
那么，如何使用decode函数呢？.好吧，我们知道第一个参数应该是ImageFormat的一个示例，所以让我们选择PNG

my_bytestring :: ByteString
my_bytestring = ... whateve

wrong_my_image = decode PNG some_bytestring
-- You could think that this read an Image from a PNG but, which image should we pick??
-- There are many instances for this. For example:
-- Readable (Image VS RGB Double) PNG  reads a PNG as a Vector of Doubles in the RGB color space
-- Readable (Image VS RGBA Word8) PNG  reads a PNG as a Vector of 8-bits unsigned integers in the RGB with alpha color space
-- Readable (Image VS Y Word16) PNG    reads a PNG as a Vector of 16-bits unsigned integers in the YUV color space
-- etc...

right_my_image_1 :: Either String (Image VS RGB Double) -- For example
right_my_image_1 = decode PNG my_bytestring

right_my_image_2 = decode PNG my_bytestring :: Either String (Image VS RGB Double)
-- Notice that you must(!!) specify the type you want to read to with explicit
-- annotations. Wheter the annotation is in two lines or one line is up to you.