[Haskell-cafe] Getting an attribute of an object

Dmitri Pissarenko mailing-lists at dapissarenko.com
Sun Feb 13 04:16:16 EST 2005


Thanks for your answer!

> Perhaps try:
> do ...
>    ciwd <- sequence classifiedImagesWithData
>    let allImages = (getImages ciwd)
>    ...
> as it seems like you're trying to gave getImages act on a value of
> type [IO (ClassifiedImage, Image)]. Applying sequence to
> classifiedImagesWithData will turn it into an IO [(ClassifiedImage,
> Image)] by doing the natural thing, from which you can pull out the
> list to apply getImages to.

I've tried that and wrote

let classifiedImagesWithData = ((return trainingSet) >>= readClassifiedImages)
ciwd <- (sequence classifiedImagesWithData)
let allImages = (getImages ciwd)

Now, I get the error

Compiling ExperimentalYaleDb ( ExperimentalYaleFaceDb.hs, interpreted )

     Couldn't match `[]' against `IO'
         Expected type: [[a]]
         Inferred type: [IO (ClassifiedImage, Image)]
     In the first argument of `sequence', namely
     In a 'do' expression: ciwd <- (sequence classifiedImagesWithData)
Failed, modules loaded: TestLik, Lik, HUnit, HUnitText, HUnitBase, HUnitLang.

at line

ciwd <- (sequence classifiedImagesWithData)

Are there any other options?

Thanks in advance

Dmitri Pissarenko

PS: Maybe the error is rooted in code parts other than those given here. In
the attachment there is the code of the main program
(ExperimentalYaleFaceDb.hs) and the function definitions (Lik.hs).
Dmitri Pissarenko
Software Engineer

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