Generics.SOP.TH

Generate generics-sop boilerplate instances using Template Haskell.

Documentation

deriveGeneric :: Name -> Q [Dec]

Generate generics-sop boilerplate for the given datatype.

This function takes the name of a datatype and generates:

• a Code instance
• a Generic instance
• a HasDatatypeInfo instance

Note that the generated code will require the TypeFamilies and DataKinds extensions to be enabled for the module.

Example: If you have the datatype

data Tree = Leaf Int | Node Tree Tree

and say

deriveGeneric ''Tree

then you get code that is equivalent to:

instance Generic Tree where

  type Code Tree = '[ '[Int], '[Tree, Tree] ]

  from (Leaf x)   = SOP (   Z (I x :* Nil))
  from (Node l r) = SOP (S (Z (I l :* I r :* Nil)))

  to (SOP    (Z (I x :* Nil)))         = Leaf x
  to (SOP (S (Z (I l :* I r :* Nil)))) = Node l r
  to _ = error "unreachable" -- to avoid GHC warnings

instance HasDatatypeInfo Tree where
  datatypeInfo _ = ADT "Main" "Tree"
    (Constructor "Leaf" :* Constructor "Node" :* Nil)

Limitations: Generation does not work for GADTs, for datatypes that involve existential quantification, for datatypes with unboxed fields.

deriveGenericOnly :: Name -> Q [Dec]

Like deriveGeneric, but omit the HasDatatypeInfo instance.