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PO
primarykey
Id
9501802
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
3
CommunityOwnedDate
CreationDate
2012-02-29T15:22:24.317
FavoriteCount
0
LastActivityDate
2012-02-29T15:34:40.063
LastEditDate
2012-02-29T15:34:40.063
LastEditorUserId
1097181
OwnerUserId
1097181
ParentId
9501625
PostTypeId
2
Score
5
ViewCount
0
LastEditorDisplayName
text
Body
The problem here is <code>area</code>'s type signature: <pre><code>area :: (Num b) => a -> Area b </code></pre> What it says is "give me an <code>a</code>, and I'll give you an <code>Area b</code> for any <code>b</code> you want; you can pick". So, for instance, I could give <code>area</code> an <code>Integer</code> and expect back an <code>Area Double</code>. Clearly, this isn't what you want! In this case, the error arises because you use <code>x*y</code>, which has type a, when b is expected — you have to give a value that works for any numeric type b, but you're giving a value that only works for one (a). If you changed <code>area</code>'s type to <code>a -> Area Integer</code>, or such, then it would work. However, I have a feeling you want the instance to be able to specify what the type of the area is. For this, you'll need to use a language extension called <a href="http://www.haskell.org/haskellwiki/GHC/Type_families" rel="noreferrer">type families</a>: <pre><code>{-# LANGUAGE TypeFamilies #-} class (Num (AreaComponent a)) => Shape a where type AreaComponent a area :: a -> Area (AreaComponent a) instance (Num a) => Shape (Rectangle a) where type AreaComponent (Rectangle a) = a area (Rectangle x y unit) = Area (x*y) unit </code></pre> This says that for every type a that's an instance of <code>Shape</code>, there is an associated type <code>AreaComponent a</code>, representing the type of each component of its area. That type is required to be an instance of <code>Num</code> by the definition of <code>Shape</code>. Another thing you could do, if all your shapes take a numeric type parameter, is to make the instances be for the type constructors of each shape, rather than the full shape types themselves: <pre><code>class Shape sh where area :: (Num a) => sh a -> Area a instance Shape Rectangle where area (Rectangle x y unit) = Area (x*y) unit </code></pre>
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