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06 Using Typeclasses

####Tables:

The need for typeclasses:

Prelude> :cd ch06
Prelude> :l naiveeq.hs 
[1 of 1] Compiling Main             ( naiveeq.hs, interpreted )
Ok, modules loaded: Main.
*Main> colorEq Red Red
True
*Main> colorEq Blue Red
False

What are typeclasses?

The keywoard to define a typeclass in Haskell is class. Unfortunately, this may be confusing for those of you coming from an object-oriented background, as we are not really defining the same thing.

class  Eq a  where
    (==), (/=) :: a -> a -> Bool

       -- Minimal complete definition:
       --     (==) or (/=)
    x /= y     =  not (x == y)
    x == y     =  not (x /= y)

Declaring typeclass instances:

Prelude> :cd ch06
Prelude> :l naiveeq.hs 
[1 of 1] Compiling Main             ( naiveeq.hs, interpreted )
Ok, modules loaded: Main.
*Main> isEqual3 Red Blue
False
*Main> isNotEqual3 Red Blue
True

Important Built-In Typeclasses:

  • show
  • putStrLn
  • read
Prelude> (read "12.4")::Double
12.4
Prelude> :t it
it :: Double

Automatic Derivation:

For many simple data types, the Haskell compiler can automatically derive instances of Read, Show, Bounded, Enum, Eq, and Ord for us.

Typeclasses at work:

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}

Refer to the wikiHaskell documentation aboutpragmas.

JSON typeclasses without overlapping instances:

# ex. 1
Prelude> :m +Control.Arrow
Prelude Control.Arrow> :t second
second :: Arrow a => a b c -> a (d, b) (d, c)
Prelude Control.Arrow> first (*2) (1, 1)
(2,1)
Prelude Control.Arrow> second (*2) (1, 1)
(1,2)

 # ex. 2
 Prelude Control.Arrow> :t (,)
(,) :: a -> b -> (a, b)
Prelude Control.Arrow> :t (,,)
(,,) :: a -> b -> c -> (a, b, c)
Prelude Control.Arrow> (,) 1 2
(1,2)

The dreaded monomorphism restriction:

[...] if GHC complains about the monomorphism restriction, we have three easy ways to address the error. Make the function's arguments explicit, instead of leaving them implicit. Give the definition an explicit type signature, instead of making the compiler infer its type. Leave the code untouched, and compile the module with the NoMonomorphismRestriction language extension enabled. This disables the monomorphism restriction.