7.16. Generic classes
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## 7.16. Generic classes

The ideas behind this extension are described in detail in "Derivable type classes", Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105. An example will give the idea:

      import Data.Generics

      class Bin a where
        toBin   :: a -> [Int]
        fromBin :: [Int] -> (a, [Int])

        toBin {| Unit |}    Unit	  = []
        toBin {| a :+: b |} (Inl x)   = 0 : toBin x
        toBin {| a :+: b |} (Inr y)   = 1 : toBin y
        toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y

        fromBin {| Unit |}    bs      = (Unit, bs)
        fromBin {| a :+: b |} (0:bs)  = (Inl x, bs')    where (x,bs') = fromBin bs
        fromBin {| a :+: b |} (1:bs)  = (Inr y, bs')    where (y,bs') = fromBin bs
        fromBin {| a :*: b |} bs	  = (x :*: y, bs'') where (x,bs' ) = fromBin bs
    							  (y,bs'') = fromBin bs'

This class declaration explains how `toBin` and `fromBin` work for arbitrary data types. They do so by giving cases for unit, product, and sum, which are defined thus in the library module `Data.Generics`:

      data Unit    = Unit
      data a :+: b = Inl a | Inr b
      data a :*: b = a :*: b

Now you can make a data type into an instance of Bin like this:

      instance (Bin a, Bin b) => Bin (a,b)
      instance Bin a => Bin [a]

That is, just leave off the "where" clause. Of course, you can put in the where clause and over-ride whichever methods you please.

### 7.16.1. Using generics

To use generics you need to

- Use the flags `-XGenerics` (to enable the extra syntax and generate extra per-data-type code), and `-package syb` (to make the `Data.Generics` module available.

- Import the module `Data.Generics` from the `syb` package. This import brings into scope the data types `Unit`, `:*:`, and `:+:`. (You don't need this import if you don't mention these types explicitly; for example, if you are simply giving instance declarations.)

### 7.16.2. Changes wrt the paper

Note that the type constructors `:+:` and `:*:` can be written infix (indeed, you can now use any operator starting in a colon as an infix type constructor). Also note that the type constructors are not exactly as in the paper (Unit instead of 1, etc). Finally, note that the syntax of the type patterns in the class declaration uses "`{|`" and "`|}`" brackets; curly braces alone would ambiguous when they appear on right hand sides (an extension we anticipate wanting).

### 7.16.3. Terminology and restrictions

Terminology. A "generic default method" in a class declaration is one that is defined using type patterns as above. A "polymorphic default method" is a default method defined as in Haskell 98. A "generic class declaration" is a class declaration with at least one generic default method.

Restrictions:

- Alas, we do not yet implement the stuff about constructor names and field labels.

- A generic class can have only one parameter; you can't have a generic multi-parameter class.

- A default method must be defined entirely using type patterns, or entirely without. So this is illegal:

        class Foo a where
          op :: a -> (a, Bool)
          op {| Unit |} Unit = (Unit, True)
          op x               = (x,    False)

  However it is perfectly OK for some methods of a generic class to have generic default methods and others to have polymorphic default methods.

- The type variable(s) in the type pattern for a generic method declaration scope over the right hand side. So this is legal (note the use of the type variable ``p'' in a type signature on the right hand side:

        class Foo a where
          op :: a -> Bool
          op {| p :*: q |} (x :*: y) = op (x :: p)
          ...

- The type patterns in a generic default method must take one of the forms:

             a :+: b
             a :*: b
             Unit

  where "a" and "b" are type variables. Furthermore, all the type patterns for a single type constructor (`:*:`, say) must be identical; they must use the same type variables. So this is illegal:

        class Foo a where
          op :: a -> Bool
          op {| a :+: b |} (Inl x) = True
          op {| p :+: q |} (Inr y) = False

  The type patterns must be identical, even in equations for different methods of the class. So this too is illegal:

        class Foo a where
          op1 :: a -> Bool
          op1 {| a :*: b |} (x :*: y) = True

          op2 :: a -> Bool
          op2 {| p :*: q |} (x :*: y) = False

  (The reason for this restriction is that we gather all the equations for a particular type constructor into a single generic instance declaration.)

- A generic method declaration must give a case for each of the three type constructors.

- The type for a generic method can be built only from:

  - Function arrows

  - Type variables

  - Tuples

  - Arbitrary types not involving type variables

  Here are some example type signatures for generic methods:

          op1 :: a -> Bool
          op2 :: Bool -> (a,Bool)
          op3 :: [Int] -> a -> a
          op4 :: [a] -> Bool

  Here, op1, op2, op3 are OK, but op4 is rejected, because it has a type variable inside a list.

  This restriction is an implementation restriction: we just haven't got around to implementing the necessary bidirectional maps over arbitrary type constructors. It would be relatively easy to add specific type constructors, such as Maybe and list, to the ones that are allowed.

- In an instance declaration for a generic class, the idea is that the compiler will fill in the methods for you, based on the generic templates. However it can only do so if

  - The instance type is simple (a type constructor applied to type variables, as in Haskell 98).

  - No constructor of the instance type has unboxed fields.

  (Of course, these things can only arise if you are already using GHC extensions.) However, you can still give an instance declarations for types which break these rules, provided you give explicit code to override any generic default methods.

The option `-ddump-deriv` dumps incomprehensible stuff giving details of what the compiler does with generic declarations.

### 7.16.4. Another example

Just to finish with, here's another example I rather like:

      class Tag a where
        nCons :: a -> Int
        nCons {| Unit |}    _ = 1
        nCons {| a :*: b |} _ = 1
        nCons {| a :+: b |} _ = nCons (bot::a) + nCons (bot::b)

        tag :: a -> Int
        tag {| Unit |}    _       = 1
        tag {| a :*: b |} _       = 1
        tag {| a :+: b |} (Inl x) = tag x
        tag {| a :+: b |} (Inr y) = nCons (bot::a) + tag y

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