[Haskell-cafe] Getting an attribute of an object

[Jules Bean]

On 10 Feb 2005, at 20:17, Dmitri Pissarenko wrote:

> Hello!
>
> I have a list of instances of the ClassifiedImage class, which is 
> defined as
> follows.
>
> data ClassifiedImage = ClassifiedImage {imageFileName :: String, 
> subjectID ::
> String}
>         deriving Show
>
> Attribute imageFileName contains a file name of a certain image. I want
> to "transform" the list [ClassifiedImage] into a list 
> [(ClassifiedImage,
> Image)], where Image is content of the file with name imageFileName.
>
> That is, I want to have a routine, which iterates through the list of
> ClassifiedImages, reads each file (with filename contained in the 
> attribute
> ClassifiedImage.imageFileName) and stores its content in a variable.
>
> I have already a function, which reads the content of a file.
>
> My idea is to use map for this task:
>
> readClassifiedImages :: [ClassifiedImage] -> [(ClassifiedImage, Image)]
> readClassifiedImages classifiedImages = do
>         return map readerFunc classifiedImages
>
> readerFunc denotes some function, which takes the attribute 
> imageFileName of a
> ClassifiedImage instance.
>
> I suppose that this readerFunc looks like shown below.
>
> readerFunc :: ClassifiedImage -> (ClassifiedImage, Image)
> readerFunc classifiedImage = (classifiedImage, fileContent)
>         where   fileName = classifiedImageFileName classifiedImage
>                 fileContent = readImage fileName
>
> classifiedImageFileName :: ClassifiedImage -> String
>
> In order for this function to work, I need to define 
> classifiedImageFileName,
> which "returns" the imageFileName attribute of an instance of 
> ClassifiedImage
> class.

This is called a selector function. Haskell defines one automatically 
with the same name as the field. imageFileName is a function 
ClassifiedImage -> String.

(ClassifiedImage is not a class, it's a datatype, by the way)

>
> How can I define this function?
>
> Can I use map for readImage function, which is in the IO domain? If 
> not, what
> tutorial can help me?
>

If you're happy to replace Image with IO image, then that will be fine.

Jules