Derives an instance of @Monoid@. It uses the product
    construction of monoids. @mappend@ on two different constructors
    is undefined.
module Data.Derive.Monoid(makeMonoid) where
import Data.Monoid hiding (First)

example :: Sample
instance Monoid a => Monoid (Sample a) where
    mempty = head [First, Second (const mempty 1) (const mempty 2), Third (const mempty 1)]
    mappend (First) (First) = First
    mappend (Second x1 x2) (Second y1 y2) = Second (mappend x1 y1) (mappend x2 y2)
    mappend (Third x1) (Third y1) = Third (mappend x1 y1)
    mappend _ _ | length [First{},Second{},Third{}] > 1 = error "Monoid.mappend: Different constructors for Sample"


import Data.Derive.DSL.DSL
import Data.Derive.Internal.Derivation

makeMonoid :: Derivation
makeMonoid = derivationDSL "Monoid" dslMonoid

dslMonoid =
    List [Instance ["Monoid"] "Monoid" (List [App "InsDecl" (List [App
    "PatBind" (List [App "PVar" (List [App "Ident" (List [String
    "mempty"])]),App "Nothing" (List []),App "UnGuardedRhs" (List [App
    "App" (List [App "Var" (List [App "UnQual" (List [App "Ident" (
    List [String "head"])])]),App "List" (List [MapCtor (Application (
    Concat (List [List [App "Con" (List [App "UnQual" (List [App
    "Ident" (List [CtorName])])])],MapField (App "Paren" (List [
    Application (List [App "Var" (List [App "UnQual" (List [App
    "Ident" (List [String "const"])])]),App "Var" (List [App "UnQual"
    (List [App "Ident" (List [String "mempty"])])]),App "Lit" (List [
    App "Int" (List [FieldIndex])])])]))])))])])]),App "BDecls" (List
    [List []])])]),App "InsDecl" (List [App "FunBind" (List [Concat (
    List [MapCtor (App "Match" (List [App "Ident" (List [String
    "mappend"]),List [App "PParen" (List [App "PApp" (List [App
    "UnQual" (List [App "Ident" (List [CtorName])]),MapField (App
    "PVar" (List [App "Ident" (List [Concat (List [String "x",ShowInt
    FieldIndex])])]))])]),App "PParen" (List [App "PApp" (List [App
    "UnQual" (List [App "Ident" (List [CtorName])]),MapField (App
    "PVar" (List [App "Ident" (List [Concat (List [String "y",ShowInt
    FieldIndex])])]))])])],App "Nothing" (List []),App "UnGuardedRhs"
    (List [Application (Concat (List [List [App "Con" (List [App
    "UnQual" (List [App "Ident" (List [CtorName])])])],MapField (App
    "Paren" (List [Application (List [App "Var" (List [App "UnQual" (
    List [App "Ident" (List [String "mappend"])])]),App "Var" (List [
    App "UnQual" (List [App "Ident" (List [Concat (List [String "x",
    ShowInt FieldIndex])])])]),App "Var" (List [App "UnQual" (List [
    App "Ident" (List [Concat (List [String "y",ShowInt FieldIndex])])
    ])])])]))]))]),App "BDecls" (List [List []])])),List [App "Match"
    (List [App "Ident" (List [String "mappend"]),List [App "PWildCard"
    (List []),App "PWildCard" (List [])],App "Nothing" (List []),App
    "GuardedRhss" (List [List [App "GuardedRhs" (List [List [App
    "Qualifier" (List [App "InfixApp" (List [App "App" (List [App
    "Var" (List [App "UnQual" (List [App "Ident" (List [String
    "length"])])]),App "List" (List [MapCtor (App "RecConstr" (List [
    App "UnQual" (List [App "Ident" (List [CtorName])]),List []]))])])
    ,App "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [
    String ">"])])]),App "Lit" (List [App "Int" (List [Int 1])])])])],
    App "App" (List [App "Var" (List [App "UnQual" (List [App "Ident"
    (List [String "error"])])]),App "Lit" (List [App "String" (List [
    Concat (List [String "Monoid.mappend: Different constructors for "
    ,DataName])])])])])]]),App "BDecls" (List [List []])])]])])])])]