Sjoerd Visscher | 19 Sep 23:19 2012

ANNOUCE: one-liner-0, SYB-like generics with constraint kinds

Hi all,

I am pleased to announce the first release of One-Liner, a package for writing short and concise generic
instances of type classes. It works a bit like Scrap-Your-Boilerplate, but it uses the new constraint
kinds instead of the Typeable class.

On hackage: http://hackage.haskell.org/package/one-liner-0
On github: https://github.com/sjoerdvisscher/one-liner

For example, this is how to write generic equality (using the All monoid):

  eqADT :: (ADT t, Constraints t Eq) => t -> t -> Bool
  eqADT s t = ctorIndex s == ctorIndex t &&
    getAll (mbuilds (For :: For Eq) (\fld -> All $ s ! fld == t ! fld) `at` s)

The code works like this: "Constraints t Eq" means it requires that all subcomponents of type t have an Eq
instance, and then values s and t are equal if they are built by the same constructor and each subcomponent
is equal.

The package is called One-Liner because the generic functions often end up as short as eqADT, especially if
there's already an appropriate Monoid or Applicative functor available.

Here are two more examples, generic put and get for the Binary package (after adding the missing Monoid
instance for Put)

  putADT :: (ADT t, Constraints t Binary) => t -> Put
  putADT t = putWord8 (toEnum (ctorIndex t)) >> gfoldMap (For :: For Binary) put

  getADT :: (ADT t, Constraints t Binary) => Get t
  getADT = do
(Continue reading)


Gmane