add an instance declaration haskell

Question

This is my function:

    bestLocByArtiest :: String -> [Review] -> Double
    bestLocByArtiest art = rev
        where gmloc = [gemZaalByArtiest art, gemFestByArtiest art, gemOpenByArtiest art]
              rev = max gmloc

This is the error i get:

    Review.hs:101:24:
Couldn't match type `[Review] -> Double' with `Review'
Expected type: [Review] -> Double
  Actual type: [[Review] -> Double] -> [[Review] -> Double]
In the expression: rev
In an equation for `bestLocByArtiest':
    bestLocByArtiest art
      = rev
      where
          gmloc = [gemZaalByArtiest art, ....]
          rev = max gmloc

So I was hoping somebody could explain me what this error means and what I have to change in my code to solve it.

EDIT By changing max to maximum i got this new error:

    Review.hs:103:21:
No instance for (Ord ([Review] -> Double))
  arising from a use of `maximum'
Possible fix:
  add an instance declaration for (Ord ([Review] -> Double))
In the expression: maximum gmloc
In an equation for `rev': rev = maximum gmloc
In an equation for `bestLocByArtiest':
    bestLocByArtiest art
      = rev
      where
          gmloc = [gemZaalByArtiest art, ....]
          rev = maximum gmloc

What does this mean?

max does maximum of two, not of a list of N elements. You want maximum. Also, you don't seem to be accepting your second argument (the [Review]). — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented Dec 18, 2013 at 17:01

Simeon Visser · Accepted Answer · 2013-12-18 17:00:37Z

1

Given that bestLocByArtiest is called with a String the type of rev needs to be [Review] -> Double. However it's actually [[Review] -> Double] -> [[Review] -> Double]. That's basically what the error message says.

So to fix it you either need to change the type signature of bestLocByArtiest or you need to modify what rev does. That involves looking at the types of max, gmloc and the individual elements in gmloc to make sure it does what you want.

answered Dec 18, 2013 at 17:00

Simeon Visser

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