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    <p>First, I am always very suspicious of claims that this or that program structure is more intuitive. Programming is counter-intuitive and always will be because people naturally think in terms of specific cases rather than general rules. Changing this requires training and practice, otherwise known as "learning to program".</p> <p>Moving on to the meat of the question, the key difference between OO classes and Haskell typeclasses is that in OO a class (even an interface class) is both a type and a template for new types (descendants). In Haskell a typeclass is <em>only</em> a template for new types. More precisely, a typeclass describes a set of types that share a common interface, but it is <em>not itself a type</em>.</p> <p>So the typeclass "Num" describes numeric types with addition, subtraction and multiplication operators. The "Integer" type is an instance of "Num", meaning that Integer is a member of the set of types that implement those operators.</p> <p>So I can write a summation function with this type:</p> <pre><code>sum :: Num a =&gt; [a] -&gt; a </code></pre> <p>The bit to to the left of the "=>" operator says that "sum" will work for any type "a" that is an instance of Num. The bit to the right says it takes a list of values of type "a" and returns a single value of type "a" as a result. So you could use it to sum a list of Integers or a list of Doubles or a list of Complex, because they are all instances of "Num". The implementation of "sum" will use the "+" operator of course, which is why you need the "Num" type constraint.</p> <p>However you cannot write this:</p> <pre><code>sum :: [Num] -&gt; Num </code></pre> <p>because "Num" is not a type.</p> <p>This distinction between type and typeclass is why we don't talk about inheritance and descendants of types in Haskell. There <em>is</em> a sort of inheritance for typeclasses: you can declare one typeclass as a descendant of another. The descendant here describes a subset of the types described by parent.</p> <p>An important consequence of all this is that you can't have heterogenous lists in Haskell. In the "sum" example you can pass it a list of integers or a list of doubles, but you cannot mix doubles and integers in the same list. This looks like a tricky restriction; how would you implement the old "cars and lorries are both types of vehicle" example? There are several answers depending on the problem you are actually trying to solve, but the general principle is that you do your indirection explicitly using first-class functions rather than implicitly using virtual functions.</p>
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    1. POExplain Type Classes in Haskell
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    1. COSuperclasses from contexts are a pretty sorry excuse for "inheritance", though, since they require you to manually write instances for all the parent classes. Not to mention bothersome issues like "why isn't Functor a superclass of Monad"...
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      1. USC. A. McCann
    2. COHaving come from an OO world before learning Haskell, I've come to think of inheritance as a pretty sorry excuse for first class functions.
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    3. COFunctor is a superclass of Monad. You are just using the wrong implementation of Monad.
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