Math.Combinatorics.Species.TH

Use Template Haskell to automatically derive species instances for user-defined data types.

Documentation

deriveDefaultSpecies :: Name -> Q [Dec]

Generate default species declarations for the given user-defined data type. To use it:

{-# LANGUAGE TemplateHaskell,
             TypeFamilies,
             DeriveDataTypeable,
             FlexibleInstances,
             UndecidableInstances #-}

data MyType = ...

$(deriveDefaultSpecies ''MyType)

Yes, you really do need all those extensions. And don't panic about the UndecidableInstances; the instances generated actually are decidable, but GHC just can't tell.

This is what you get:

• An Enumerable instance for MyType (and various other supporting things like a code and an ASTFunctor instance if your data type is recursive)
• A declaration of myType :: Species s => s (the same name as the type constructor but with the first letter lowercased)

You can then use myType in any species expression, or as input to any function expecting a species. For example, to count your data type's distinct shapes, you can do

take 10 . unlabeled $ myType

deriveSpecies :: Name -> SpeciesAST -> Q [Dec]

Like deriveDefaultSpecies, except that you specify the species expression that your data type should be isomorphic to. Note: this is currently experimental (read: bug-ridden).