CODEWITHSUNDEEP
haskelltypestypeclass
Aneesh Dandime
Aneesh Dandime

Reputation: 123

Type classes - Brief Explanation

I am new to Haskell and i come from c++ background.

I am having some trouble understanding the relation between types and type classes.

From what i understand type classes specify a bunch of functions. Like the Eq type class specifies == and /=.

But now suppose i have a type t, What does it mean when i say that t is an instance of Eq type class. Does it simply mean that t supports and implements the functions specified by Eq ?

Upvotes: 2

Views: 78

Answers (1)

chi
chi

Reputation: 116139

Does it simply mean that t supports and implements the functions specified by Eq ?

Yes, precisely.

It simply means that there's an instance of Eq t which defines the operator (==), as required by Eq. You can therefore use said operator on type t in your code.

A silly example:

data MyPair = P Int String

instance Eq MyPair where
   (P i1 s1) == (P i2 s2) =   -- we define equality ...
      i1==i2 && s1==S2        -- ... as component-wise equality

testTrue :: Bool
testTrue = (P (1+1) "aa") == (P 2 ("a"++"a"))

In the example above, the common idiom would actually be

data MyPair = P Int String deriving Eq

which automatically derives the instance -- so there's no need to write it explicitly.

Upvotes: 6

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