Synnefo » snf-ganeti

{-# LANGUAGE ExistentialQuantification, ParallelListComp, TemplateHaskell #-}

{-| TemplateHaskell helper for Ganeti Haskell code.

As TemplateHaskell require that splices be defined in a separate

module, we combine all the TemplateHaskell functionality that HTools

needs in this module (except the one for unittests).

-}

{-

Copyright (C) 2011, 2012 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.THH ( declareSADT

                  , declareLADT

                  , declareILADT

                  , declareIADT

                  , makeJSONInstance

                  , deCamelCase

                  , genOpID

                  , genAllConstr

                  , genAllOpIDs

                  , PyValue(..)

                  , PyValueEx(..)

                  , OpCodeDescriptor

                  , genOpCode

                  , genStrOfOp

                  , genStrOfKey

                  , genLuxiOp

                  , Field (..)

                  , simpleField

                  , withDoc

                  , defaultField

                  , optionalField

                  , optionalNullSerField

                  , renameField

                  , customField

                  , timeStampFields

                  , uuidFields

                  , serialFields

                  , tagsFields

                  , TagSet

                  , buildObject

                  , buildObjectSerialisation

                  , buildParam

                  , DictObject(..)

                  , genException

                  , excErrMsg

                  ) where

import Control.Monad (liftM)

import Data.Char

import Data.List

import qualified Data.Set as Set

import Language.Haskell.TH

import qualified Text.JSON as JSON

import Text.JSON.Pretty (pp_value)

import Ganeti.JSON

import Data.Maybe

import Data.Functor ((<$>))

-- * Exported types

-- | Class of objects that can be converted to 'JSObject'

-- lists-format.

class DictObject a where

  toDict :: a -> [(String, JSON.JSValue)]

-- | Optional field information.

data OptionalType

  = NotOptional           -- ^ Field is not optional

  | OptionalOmitNull      -- ^ Field is optional, null is not serialised

  | OptionalSerializeNull -- ^ Field is optional, null is serialised

  deriving (Show, Eq)

-- | Serialised field data type.

data Field = Field { fieldName        :: String

                   , fieldType        :: Q Type

                   , fieldRead        :: Maybe (Q Exp)

                   , fieldShow        :: Maybe (Q Exp)

                   , fieldExtraKeys   :: [String]

                   , fieldDefault     :: Maybe (Q Exp)

                   , fieldConstr      :: Maybe String

                   , fieldIsOptional  :: OptionalType

                   , fieldDoc         :: String

                   }

-- | Generates a simple field.

simpleField :: String -> Q Type -> Field

simpleField fname ftype =

  Field { fieldName        = fname

        , fieldType        = ftype

        , fieldRead        = Nothing

        , fieldShow        = Nothing

        , fieldExtraKeys   = []

        , fieldDefault     = Nothing

        , fieldConstr      = Nothing

        , fieldIsOptional  = NotOptional

        , fieldDoc         = ""

        }

withDoc :: String -> Field -> Field

withDoc doc field =

  field { fieldDoc = doc }

-- | Sets the renamed constructor field.

renameField :: String -> Field -> Field

renameField constrName field = field { fieldConstr = Just constrName }

-- | Sets the default value on a field (makes it optional with a

-- default value).

defaultField :: Q Exp -> Field -> Field

defaultField defval field = field { fieldDefault = Just defval }

-- | Marks a field optional (turning its base type into a Maybe).

optionalField :: Field -> Field

optionalField field = field { fieldIsOptional = OptionalOmitNull }

-- | Marks a field optional (turning its base type into a Maybe), but

-- with 'Nothing' serialised explicitly as /null/.

optionalNullSerField :: Field -> Field

optionalNullSerField field = field { fieldIsOptional = OptionalSerializeNull }

-- | Sets custom functions on a field.

customField :: Name      -- ^ The name of the read function

            -> Name      -- ^ The name of the show function

            -> [String]  -- ^ The name of extra field keys

            -> Field     -- ^ The original field

            -> Field     -- ^ Updated field

customField readfn showfn extra field =

  field { fieldRead = Just (varE readfn), fieldShow = Just (varE showfn)

        , fieldExtraKeys = extra }

-- | Computes the record name for a given field, based on either the

-- string value in the JSON serialisation or the custom named if any

-- exists.

fieldRecordName :: Field -> String

fieldRecordName (Field { fieldName = name, fieldConstr = alias }) =

  fromMaybe (camelCase name) alias

-- | Computes the preferred variable name to use for the value of this

-- field. If the field has a specific constructor name, then we use a

-- first-letter-lowercased version of that; otherwise, we simply use

-- the field name. See also 'fieldRecordName'.

fieldVariable :: Field -> String

fieldVariable f =

  case (fieldConstr f) of

    Just name -> ensureLower name

    _ -> map (\c -> if c == '-' then '_' else c) $ fieldName f

-- | Compute the actual field type (taking into account possible

-- optional status).

actualFieldType :: Field -> Q Type

actualFieldType f | fieldIsOptional f /= NotOptional = [t| Maybe $t |]

                  | otherwise = t

                  where t = fieldType f

-- | Checks that a given field is not optional (for object types or

-- fields which should not allow this case).

checkNonOptDef :: (Monad m) => Field -> m ()

checkNonOptDef (Field { fieldIsOptional = OptionalOmitNull

                      , fieldName = name }) =

  fail $ "Optional field " ++ name ++ " used in parameter declaration"

checkNonOptDef (Field { fieldIsOptional = OptionalSerializeNull

                      , fieldName = name }) =

  fail $ "Optional field " ++ name ++ " used in parameter declaration"

checkNonOptDef (Field { fieldDefault = (Just _), fieldName = name }) =

  fail $ "Default field " ++ name ++ " used in parameter declaration"

checkNonOptDef _ = return ()

-- | Produces the expression that will de-serialise a given

-- field. Since some custom parsing functions might need to use the

-- entire object, we do take and pass the object to any custom read

-- functions.

loadFn :: Field   -- ^ The field definition

       -> Q Exp   -- ^ The value of the field as existing in the JSON message

       -> Q Exp   -- ^ The entire object in JSON object format

       -> Q Exp   -- ^ Resulting expression

loadFn (Field { fieldRead = Just readfn }) expr o = [| $expr >>= $readfn $o |]

loadFn _ expr _ = expr

-- * Common field declarations

-- | Timestamp fields description.

timeStampFields :: [Field]

timeStampFields =

    [ defaultField [| 0::Double |] $ simpleField "ctime" [t| Double |]

    , defaultField [| 0::Double |] $ simpleField "mtime" [t| Double |]

    ]

-- | Serial number fields description.

serialFields :: [Field]

serialFields =

    [ renameField  "Serial" $ simpleField "serial_no" [t| Int |] ]

-- | UUID fields description.

uuidFields :: [Field]

uuidFields = [ simpleField "uuid" [t| String |] ]

-- | Tag set type alias.

type TagSet = Set.Set String

-- | Tag field description.

tagsFields :: [Field]

tagsFields = [ defaultField [| Set.empty |] $

               simpleField "tags" [t| TagSet |] ]

-- * Internal types

-- | A simple field, in constrast to the customisable 'Field' type.

type SimpleField = (String, Q Type)

-- | A definition for a single constructor for a simple object.

type SimpleConstructor = (String, [SimpleField])

-- | A definition for ADTs with simple fields.

type SimpleObject = [SimpleConstructor]

-- | A type alias for an opcode constructor of a regular object.

type OpCodeConstructor = (String, Q Type, String, [Field], String)

-- | A type alias for a Luxi constructor of a regular object.

type LuxiConstructor = (String, [Field])

-- * Helper functions

-- | Ensure first letter is lowercase.

--

-- Used to convert type name to function prefix, e.g. in @data Aa ->

-- aaToRaw@.

ensureLower :: String -> String

ensureLower [] = []

ensureLower (x:xs) = toLower x:xs

-- | Ensure first letter is uppercase.

--

-- Used to convert constructor name to component

ensureUpper :: String -> String

ensureUpper [] = []

ensureUpper (x:xs) = toUpper x:xs

-- | Helper for quoted expressions.

varNameE :: String -> Q Exp

varNameE = varE . mkName

-- | showJSON as an expression, for reuse.

showJSONE :: Q Exp

showJSONE = varE 'JSON.showJSON

-- | makeObj as an expression, for reuse.

makeObjE :: Q Exp

makeObjE = varE 'JSON.makeObj

-- | fromObj (Ganeti specific) as an expression, for reuse.

fromObjE :: Q Exp

fromObjE = varE 'fromObj

-- | ToRaw function name.

toRawName :: String -> Name

toRawName = mkName . (++ "ToRaw") . ensureLower

-- | FromRaw function name.

fromRawName :: String -> Name

fromRawName = mkName . (++ "FromRaw") . ensureLower

-- | Converts a name to it's varE\/litE representations.

reprE :: Either String Name -> Q Exp

reprE = either stringE varE

-- | Smarter function application.

--

-- This does simply f x, except that if is 'id', it will skip it, in

-- order to generate more readable code when using -ddump-splices.

appFn :: Exp -> Exp -> Exp

appFn f x | f == VarE 'id = x

          | otherwise = AppE f x

-- | Builds a field for a normal constructor.

buildConsField :: Q Type -> StrictTypeQ

buildConsField ftype = do

  ftype' <- ftype

  return (NotStrict, ftype')

-- | Builds a constructor based on a simple definition (not field-based).

buildSimpleCons :: Name -> SimpleObject -> Q Dec

buildSimpleCons tname cons = do

  decl_d <- mapM (\(cname, fields) -> do

                    fields' <- mapM (buildConsField . snd) fields

                    return $ NormalC (mkName cname) fields') cons

  return $ DataD [] tname [] decl_d [''Show, ''Eq]

-- | Generate the save function for a given type.

genSaveSimpleObj :: Name                            -- ^ Object type

                 -> String                          -- ^ Function name

                 -> SimpleObject                    -- ^ Object definition

                 -> (SimpleConstructor -> Q Clause) -- ^ Constructor save fn

                 -> Q (Dec, Dec)

genSaveSimpleObj tname sname opdefs fn = do

  let sigt = AppT (AppT ArrowT (ConT tname)) (ConT ''JSON.JSValue)

      fname = mkName sname

  cclauses <- mapM fn opdefs

  return $ (SigD fname sigt, FunD fname cclauses)

-- * Template code for simple raw type-equivalent ADTs

-- | Generates a data type declaration.

--

-- The type will have a fixed list of instances.

strADTDecl :: Name -> [String] -> Dec

strADTDecl name constructors =

  DataD [] name []

          (map (flip NormalC [] . mkName) constructors)

          [''Show, ''Eq, ''Enum, ''Bounded, ''Ord]

-- | Generates a toRaw function.

--

-- This generates a simple function of the form:

--

-- @

-- nameToRaw :: Name -> /traw/

-- nameToRaw Cons1 = var1

-- nameToRaw Cons2 = \"value2\"

-- @

genToRaw :: Name -> Name -> Name -> [(String, Either String Name)] -> Q [Dec]

genToRaw traw fname tname constructors = do

  let sigt = AppT (AppT ArrowT (ConT tname)) (ConT traw)

  -- the body clauses, matching on the constructor and returning the

  -- raw value

  clauses <- mapM  (\(c, v) -> clause [recP (mkName c) []]

                             (normalB (reprE v)) []) constructors

  return [SigD fname sigt, FunD fname clauses]

-- | Generates a fromRaw function.

--

-- The function generated is monadic and can fail parsing the

-- raw value. It is of the form:

--

-- @

-- nameFromRaw :: (Monad m) => /traw/ -> m Name

-- nameFromRaw s | s == var1       = Cons1

--               | s == \"value2\" = Cons2

--               | otherwise = fail /.../

-- @

genFromRaw :: Name -> Name -> Name -> [(String, Either String Name)] -> Q [Dec]

genFromRaw traw fname tname constructors = do

  -- signature of form (Monad m) => String -> m $name

  sigt <- [t| (Monad m) => $(conT traw) -> m $(conT tname) |]

  -- clauses for a guarded pattern

  let varp = mkName "s"

      varpe = varE varp

  clauses <- mapM (\(c, v) -> do

                     -- the clause match condition

                     g <- normalG [| $varpe == $(reprE v) |]

                     -- the clause result

                     r <- [| return $(conE (mkName c)) |]

                     return (g, r)) constructors

  -- the otherwise clause (fallback)

  oth_clause <- do

    g <- normalG [| otherwise |]

    r <- [|fail ("Invalid string value for type " ++

                 $(litE (stringL (nameBase tname))) ++ ": " ++ show $varpe) |]

    return (g, r)

  let fun = FunD fname [Clause [VarP varp]

                        (GuardedB (clauses++[oth_clause])) []]

  return [SigD fname sigt, fun]

-- | Generates a data type from a given raw format.

--

-- The format is expected to multiline. The first line contains the

-- type name, and the rest of the lines must contain two words: the

-- constructor name and then the string representation of the

-- respective constructor.

--

-- The function will generate the data type declaration, and then two

-- functions:

--

-- * /name/ToRaw, which converts the type to a raw type

--

-- * /name/FromRaw, which (monadically) converts from a raw type to the type

--

-- Note that this is basically just a custom show\/read instance,

-- nothing else.

declareADT

  :: (a -> Either String Name) -> Name -> String -> [(String, a)] -> Q [Dec]

declareADT fn traw sname cons = do

  let name = mkName sname

      ddecl = strADTDecl name (map fst cons)

      -- process cons in the format expected by genToRaw

      cons' = map (\(a, b) -> (a, fn b)) cons

  toraw <- genToRaw traw (toRawName sname) name cons'

  fromraw <- genFromRaw traw (fromRawName sname) name cons'

  return $ ddecl:toraw ++ fromraw

declareLADT :: Name -> String -> [(String, String)] -> Q [Dec]

declareLADT = declareADT Left

declareILADT :: String -> [(String, Int)] -> Q [Dec]

declareILADT sname cons = do

  consNames <- sequence [ newName ('_':n) | (n, _) <- cons ]

  consFns <- concat <$> sequence

             [ do sig <- sigD n [t| Int |]

                  let expr = litE (IntegerL (toInteger i))

                  fn <- funD n [clause [] (normalB expr) []]

                  return [sig, fn]

             | n <- consNames

             | (_, i) <- cons ]

  let cons' = [ (n, n') | (n, _) <- cons | n' <- consNames ]

  (consFns ++) <$> declareADT Right ''Int sname cons'

declareIADT :: String -> [(String, Name)] -> Q [Dec]

declareIADT = declareADT Right ''Int

declareSADT :: String -> [(String, Name)] -> Q [Dec]

declareSADT = declareADT Right ''String

-- | Creates the showJSON member of a JSON instance declaration.

--

-- This will create what is the equivalent of:

--

-- @

-- showJSON = showJSON . /name/ToRaw

-- @

--

-- in an instance JSON /name/ declaration

genShowJSON :: String -> Q Dec

genShowJSON name = do

  body <- [| JSON.showJSON . $(varE (toRawName name)) |]

  return $ FunD 'JSON.showJSON [Clause [] (NormalB body) []]

-- | Creates the readJSON member of a JSON instance declaration.

--

-- This will create what is the equivalent of:

--

-- @

-- readJSON s = case readJSON s of

--                Ok s' -> /name/FromRaw s'

--                Error e -> Error /description/

-- @

--

-- in an instance JSON /name/ declaration

genReadJSON :: String -> Q Dec

genReadJSON name = do

  let s = mkName "s"

  body <- [| case JSON.readJSON $(varE s) of

               JSON.Ok s' -> $(varE (fromRawName name)) s'

               JSON.Error e ->

                   JSON.Error $ "Can't parse raw value for type " ++

                           $(stringE name) ++ ": " ++ e ++ " from " ++

                           show $(varE s)

           |]

  return $ FunD 'JSON.readJSON [Clause [VarP s] (NormalB body) []]

-- | Generates a JSON instance for a given type.

--

-- This assumes that the /name/ToRaw and /name/FromRaw functions

-- have been defined as by the 'declareSADT' function.

makeJSONInstance :: Name -> Q [Dec]

makeJSONInstance name = do

  let base = nameBase name

  showJ <- genShowJSON base

  readJ <- genReadJSON base

  return [InstanceD [] (AppT (ConT ''JSON.JSON) (ConT name)) [readJ,showJ]]

-- * Template code for opcodes

-- | Transforms a CamelCase string into an_underscore_based_one.

deCamelCase :: String -> String

deCamelCase =

    intercalate "_" . map (map toUpper) . groupBy (\_ b -> not $ isUpper b)

-- | Transform an underscore_name into a CamelCase one.

camelCase :: String -> String

camelCase = concatMap (ensureUpper . drop 1) .

            groupBy (\_ b -> b /= '_' && b /= '-') . ('_':)

-- | Computes the name of a given constructor.

constructorName :: Con -> Q Name

constructorName (NormalC name _) = return name

constructorName (RecC name _)    = return name

constructorName x                = fail $ "Unhandled constructor " ++ show x

-- | Extract all constructor names from a given type.

reifyConsNames :: Name -> Q [String]

reifyConsNames name = do

  reify_result <- reify name

  case reify_result of

    TyConI (DataD _ _ _ cons _) -> mapM (liftM nameBase . constructorName) cons

    o -> fail $ "Unhandled name passed to reifyConsNames, expected\

                \ type constructor but got '" ++ show o ++ "'"

-- | Builds the generic constructor-to-string function.

--

-- This generates a simple function of the following form:

--

-- @

-- fname (ConStructorOne {}) = trans_fun("ConStructorOne")

-- fname (ConStructorTwo {}) = trans_fun("ConStructorTwo")

-- @

--

-- This builds a custom list of name\/string pairs and then uses

-- 'genToRaw' to actually generate the function.

genConstrToStr :: (String -> String) -> Name -> String -> Q [Dec]

genConstrToStr trans_fun name fname = do

  cnames <- reifyConsNames name

  let svalues = map (Left . trans_fun) cnames

  genToRaw ''String (mkName fname) name $ zip cnames svalues

-- | Constructor-to-string for OpCode.

genOpID :: Name -> String -> Q [Dec]

genOpID = genConstrToStr deCamelCase

-- | Builds a list with all defined constructor names for a type.

--

-- @

-- vstr :: String

-- vstr = [...]

-- @

--

-- Where the actual values of the string are the constructor names

-- mapped via @trans_fun@.

genAllConstr :: (String -> String) -> Name -> String -> Q [Dec]

genAllConstr trans_fun name vstr = do

  cnames <- reifyConsNames name

  let svalues = sort $ map trans_fun cnames

      vname = mkName vstr

      sig = SigD vname (AppT ListT (ConT ''String))

      body = NormalB (ListE (map (LitE . StringL) svalues))

  return $ [sig, ValD (VarP vname) body []]

-- | Generates a list of all defined opcode IDs.

genAllOpIDs :: Name -> String -> Q [Dec]

genAllOpIDs = genAllConstr deCamelCase

-- | OpCode parameter (field) type.

type OpParam = (String, Q Type, Q Exp)

-- * Python code generation

-- | Converts Haskell values into Python values

--

-- This is necessary for the default values of opcode parameters and

-- return values.  For example, if a default value or return type is a

-- Data.Map, then it must be shown as a Python dictioanry.

class PyValue a where

  showValue :: a -> String

-- | Encapsulates Python default values

data PyValueEx = forall a. PyValue a => PyValueEx a

instance PyValue PyValueEx where

  showValue (PyValueEx x) = showValue x

-- | Transfers opcode data between the opcode description (through

-- @genOpCode@) and the Python code generation functions.

type OpCodeDescriptor =

  (String, String, String, [String],

   [String], [Maybe PyValueEx], [String], String)

-- | Strips out the module name

--

-- @

-- pyBaseName "Data.Map" = "Map"

-- @

pyBaseName :: String -> String

pyBaseName str =

  case span (/= '.') str of

    (x, []) -> x

    (_, _:x) -> pyBaseName x

-- | Converts a Haskell type name into a Python type name.

--

-- @

-- pyTypename "Bool" = "ht.TBool"

-- @

pyTypeName :: Show a => a -> String

pyTypeName name =

  "ht.T" ++ (case pyBaseName (show name) of

                "()" -> "None"

                "Map" -> "DictOf"

                "Set" -> "SetOf"

                "ListSet" -> "SetOf"

                "Either" -> "Or"

                "GenericContainer" -> "DictOf"

                "JSValue" -> "Any"

                "JSObject" -> "Object"

                str -> str)

-- | Converts a Haskell type into a Python type.

--

-- @

-- pyType [Int] = "ht.TListOf(ht.TInt)"

-- @

pyType :: Type -> Q String

pyType (AppT typ1 typ2) =

  do t <- pyCall typ1 typ2

     return $ t ++ ")"

pyType (ConT name) = return (pyTypeName name)

pyType ListT = return "ht.TListOf"

pyType (TupleT 0) = return "ht.TNone"

pyType (TupleT _) = return "ht.TTupleOf"

pyType typ = error $ "unhandled case for type " ++ show typ

-- | Converts a Haskell type application into a Python type.

--

-- @

-- Maybe Int = "ht.TMaybe(ht.TInt)"

-- @

pyCall :: Type -> Type -> Q String

pyCall (AppT typ1 typ2) arg =

  do t <- pyCall typ1 typ2

     targ <- pyType arg

     return $ t ++ ", " ++ targ

pyCall typ1 typ2 =

  do t1 <- pyType typ1

     t2 <- pyType typ2

     return $ t1 ++ "(" ++ t2

-- | @pyType opt typ@ converts Haskell type @typ@ into a Python type,

-- where @opt@ determines if the converted type is optional (i.e.,

-- Maybe).

--

-- @

-- pyType False [Int] = "ht.TListOf(ht.TInt)" (mandatory)

-- pyType True [Int] = "ht.TMaybe(ht.TListOf(ht.TInt))" (optional)

-- @

pyOptionalType :: Bool -> Type -> Q String

pyOptionalType opt typ

  | opt = do t <- pyType typ

             return $ "ht.TMaybe(" ++ t ++ ")"

  | otherwise = pyType typ

-- | Optionally encapsulates default values in @PyValueEx@.

--

-- @maybeApp exp typ@ returns a quoted expression that encapsulates

-- the default value @exp@ of an opcode parameter cast to @typ@ in a

-- @PyValueEx@, if @exp@ is @Just@.  Otherwise, it returns a quoted

-- expression with @Nothing@.

maybeApp :: Maybe (Q Exp) -> Q Type -> Q Exp

maybeApp Nothing _ =

  [| Nothing |]

maybeApp (Just expr) typ =

  [| Just ($(conE (mkName "PyValueEx")) ($expr :: $typ)) |]

-- | Generates a Python type according to whether the field is

-- optional

genPyType :: OptionalType -> Q Type -> Q ExpQ

genPyType opt typ =

  do t <- typ

     stringE <$> pyOptionalType (opt /= NotOptional) t

-- | Generates Python types from opcode parameters.

genPyTypes :: [Field] -> Q ExpQ

genPyTypes fs =

  listE <$> mapM (\f -> genPyType (fieldIsOptional f) (fieldType f)) fs

-- | Generates Python default values from opcode parameters.

genPyDefaults :: [Field] -> ExpQ

genPyDefaults fs =

  listE $ map (\f -> maybeApp (fieldDefault f) (fieldType f)) fs

-- | Generates a Haskell function call to "showPyClass" with the

-- necessary information on how to build the Python class string.

pyClass :: OpCodeConstructor -> ExpQ

pyClass (consName, consType, consDoc, consFields, consDscField) =

  do let pyClassVar = varNameE "showPyClass"

         consName' = stringE consName

     consType' <- genPyType NotOptional consType

     let consDoc' = stringE consDoc

         consFieldNames = listE $ map (stringE . fieldName) consFields

         consFieldDocs = listE $ map (stringE . fieldDoc) consFields

     consFieldTypes <- genPyTypes consFields

     let consFieldDefaults = genPyDefaults consFields

     [| ($consName',

         $consType',

         $consDoc',

         $consFieldNames,

         $consFieldTypes,

         $consFieldDefaults,

         $consFieldDocs,

         consDscField) |]

-- | Generates a function called "pyClasses" that holds the list of

-- all the opcode descriptors necessary for generating the Python

-- opcodes.

pyClasses :: [OpCodeConstructor] -> Q [Dec]

pyClasses cons =

  do let name = mkName "pyClasses"

         sig = SigD name (AppT ListT (ConT ''OpCodeDescriptor))

     fn <- FunD name <$> (:[]) <$> declClause cons

     return [sig, fn]

  where declClause c =

          clause [] (normalB (ListE <$> mapM pyClass c)) []

-- | Converts from an opcode constructor to a Luxi constructor.

opcodeConsToLuxiCons :: (a, b, c, d, e) -> (a, d)

opcodeConsToLuxiCons (x, _, _, y, _) = (x, y)

-- | Generates the OpCode data type.

--

-- This takes an opcode logical definition, and builds both the

-- datatype and the JSON serialisation out of it. We can't use a

-- generic serialisation since we need to be compatible with Ganeti's

-- own, so we have a few quirks to work around.

genOpCode :: String              -- ^ Type name to use

          -> [OpCodeConstructor] -- ^ Constructor name and parameters

          -> Q [Dec]

genOpCode name cons = do

  let tname = mkName name

  decl_d <- mapM (\(cname, _, _, fields, _) -> do

                    -- we only need the type of the field, without Q

                    fields' <- mapM (fieldTypeInfo "op") fields

                    return $ RecC (mkName cname) fields')

            cons

  let declD = DataD [] tname [] decl_d [''Show, ''Eq]

  let (allfsig, allffn) = genAllOpFields "allOpFields" cons

  save_decs <- genSaveOpCode tname "saveOpCode" "toDictOpCode"

               (map opcodeConsToLuxiCons cons) saveConstructor True

  (loadsig, loadfn) <- genLoadOpCode cons

  pyDecls <- pyClasses cons

  return $ [declD, allfsig, allffn, loadsig, loadfn] ++ save_decs ++ pyDecls

-- | Generates the function pattern returning the list of fields for a

-- given constructor.

genOpConsFields :: OpCodeConstructor -> Clause

genOpConsFields (cname, _, _, fields, _) =

  let op_id = deCamelCase cname

      fvals = map (LitE . StringL) . sort . nub $

              concatMap (\f -> fieldName f:fieldExtraKeys f) fields

  in Clause [LitP (StringL op_id)] (NormalB $ ListE fvals) []

-- | Generates a list of all fields of an opcode constructor.

genAllOpFields  :: String              -- ^ Function name

                -> [OpCodeConstructor] -- ^ Object definition

                -> (Dec, Dec)

genAllOpFields sname opdefs =

  let cclauses = map genOpConsFields opdefs

      other = Clause [WildP] (NormalB (ListE [])) []

      fname = mkName sname

      sigt = AppT  (AppT ArrowT (ConT ''String)) (AppT ListT (ConT ''String))

  in (SigD fname sigt, FunD fname (cclauses++[other]))

-- | Generates the \"save\" clause for an entire opcode constructor.

--

-- This matches the opcode with variables named the same as the

-- constructor fields (just so that the spliced in code looks nicer),

-- and passes those name plus the parameter definition to 'saveObjectField'.

saveConstructor :: LuxiConstructor -- ^ The constructor

                -> Q Clause        -- ^ Resulting clause

saveConstructor (sname, fields) = do

  let cname = mkName sname

  fnames <- mapM (newName . fieldVariable) fields

  let pat = conP cname (map varP fnames)

  let felems = map (uncurry saveObjectField) (zip fnames fields)

      -- now build the OP_ID serialisation

      opid = [| [( $(stringE "OP_ID"),

                   JSON.showJSON $(stringE . deCamelCase $ sname) )] |]

      flist = listE (opid:felems)

      -- and finally convert all this to a json object

      flist' = [| concat $flist |]

  clause [pat] (normalB flist') []

-- | Generates the main save opcode function.

--

-- This builds a per-constructor match clause that contains the

-- respective constructor-serialisation code.

genSaveOpCode :: Name                          -- ^ Object ype

              -> String                        -- ^ To 'JSValue' function name

              -> String                        -- ^ To 'JSObject' function name

              -> [LuxiConstructor]             -- ^ Object definition

              -> (LuxiConstructor -> Q Clause) -- ^ Constructor save fn

              -> Bool                          -- ^ Whether to generate

                                               -- obj or just a

                                               -- list\/tuple of values

              -> Q [Dec]

genSaveOpCode tname jvalstr tdstr opdefs fn gen_object = do

  tdclauses <- mapM fn opdefs

  let typecon = ConT tname

      jvalname = mkName jvalstr

      jvalsig = AppT  (AppT ArrowT typecon) (ConT ''JSON.JSValue)

      tdname = mkName tdstr

  tdsig <- [t| $(return typecon) -> [(String, JSON.JSValue)] |]

  jvalclause <- if gen_object

                  then [| $makeObjE . $(varE tdname) |]

                  else [| JSON.showJSON . map snd . $(varE tdname) |]

  return [ SigD tdname tdsig

         , FunD tdname tdclauses

         , SigD jvalname jvalsig

         , ValD (VarP jvalname) (NormalB jvalclause) []]

-- | Generates load code for a single constructor of the opcode data type.

loadConstructor :: OpCodeConstructor -> Q Exp

loadConstructor (sname, _, _, fields, _) = do

  let name = mkName sname

  fbinds <- mapM loadObjectField fields

  let (fnames, fstmts) = unzip fbinds

  let cval = foldl (\accu fn -> AppE accu (VarE fn)) (ConE name) fnames

      fstmts' = fstmts ++ [NoBindS (AppE (VarE 'return) cval)]

  return $ DoE fstmts'

-- | Generates the loadOpCode function.

genLoadOpCode :: [OpCodeConstructor] -> Q (Dec, Dec)

genLoadOpCode opdefs = do

  let fname = mkName "loadOpCode"

      arg1 = mkName "v"

      objname = mkName "o"

      opid = mkName "op_id"

  st1 <- bindS (varP objname) [| liftM JSON.fromJSObject

                                 (JSON.readJSON $(varE arg1)) |]

  st2 <- bindS (varP opid) [| $fromObjE $(varE objname) $(stringE "OP_ID") |]

  -- the match results (per-constructor blocks)

  mexps <- mapM loadConstructor opdefs

  fails <- [| fail $ "Unknown opcode " ++ $(varE opid) |]

  let mpats = map (\(me, (consName, _, _, _, _)) ->

                       let mp = LitP . StringL . deCamelCase $ consName

                       in Match mp (NormalB me) []

                  ) $ zip mexps opdefs

      defmatch = Match WildP (NormalB fails) []

      cst = NoBindS $ CaseE (VarE opid) $ mpats++[defmatch]

      body = DoE [st1, st2, cst]

  sigt <- [t| JSON.JSValue -> JSON.Result $(conT (mkName "OpCode")) |]

  return $ (SigD fname sigt, FunD fname [Clause [VarP arg1] (NormalB body) []])

-- * Template code for luxi

-- | Constructor-to-string for LuxiOp.

genStrOfOp :: Name -> String -> Q [Dec]

genStrOfOp = genConstrToStr id

-- | Constructor-to-string for MsgKeys.

genStrOfKey :: Name -> String -> Q [Dec]

genStrOfKey = genConstrToStr ensureLower

-- | Generates the LuxiOp data type.

--

-- This takes a Luxi operation definition and builds both the

-- datatype and the function transforming the arguments to JSON.

-- We can't use anything less generic, because the way different

-- operations are serialized differs on both parameter- and top-level.

--

-- There are two things to be defined for each parameter:

--

-- * name

--

-- * type

--

genLuxiOp :: String -> [LuxiConstructor] -> Q [Dec]

genLuxiOp name cons = do

  let tname = mkName name

  decl_d <- mapM (\(cname, fields) -> do

                    -- we only need the type of the field, without Q

                    fields' <- mapM actualFieldType fields

                    let fields'' = zip (repeat NotStrict) fields'

                    return $ NormalC (mkName cname) fields'')

            cons

  let declD = DataD [] (mkName name) [] decl_d [''Show, ''Eq]

  save_decs <- genSaveOpCode tname "opToArgs" "opToDict"

               cons saveLuxiConstructor False

  req_defs <- declareSADT "LuxiReq" .

              map (\(str, _) -> ("Req" ++ str, mkName ("luxiReq" ++ str))) $

                  cons

  return $ declD:save_decs ++ req_defs

-- | Generates the \"save\" clause for entire LuxiOp constructor.

saveLuxiConstructor :: LuxiConstructor -> Q Clause

saveLuxiConstructor (sname, fields) = do

  let cname = mkName sname

  fnames <- mapM (newName . fieldVariable) fields

  let pat = conP cname (map varP fnames)

  let felems = map (uncurry saveObjectField) (zip fnames fields)

      flist = [| concat $(listE felems) |]

  clause [pat] (normalB flist) []

-- * "Objects" functionality

-- | Extract the field's declaration from a Field structure.

fieldTypeInfo :: String -> Field -> Q (Name, Strict, Type)

fieldTypeInfo field_pfx fd = do

  t <- actualFieldType fd

  let n = mkName . (field_pfx ++) . fieldRecordName $ fd

  return (n, NotStrict, t)

-- | Build an object declaration.

buildObject :: String -> String -> [Field] -> Q [Dec]

buildObject sname field_pfx fields = do

  let name = mkName sname

  fields_d <- mapM (fieldTypeInfo field_pfx) fields

  let decl_d = RecC name fields_d

  let declD = DataD [] name [] [decl_d] [''Show, ''Eq]

  ser_decls <- buildObjectSerialisation sname fields

  return $ declD:ser_decls

-- | Generates an object definition: data type and its JSON instance.

buildObjectSerialisation :: String -> [Field] -> Q [Dec]

buildObjectSerialisation sname fields = do

  let name = mkName sname

  savedecls <- genSaveObject saveObjectField sname fields

  (loadsig, loadfn) <- genLoadObject loadObjectField sname fields

  shjson <- objectShowJSON sname

  rdjson <- objectReadJSON sname

  let instdecl = InstanceD [] (AppT (ConT ''JSON.JSON) (ConT name))

                 [rdjson, shjson]

  return $ savedecls ++ [loadsig, loadfn, instdecl]

-- | The toDict function name for a given type.

toDictName :: String -> Name

toDictName sname = mkName ("toDict" ++ sname)

-- | Generates the save object functionality.

genSaveObject :: (Name -> Field -> Q Exp)

              -> String -> [Field] -> Q [Dec]

genSaveObject save_fn sname fields = do

  let name = mkName sname

  fnames <- mapM (newName . fieldVariable) fields

  let pat = conP name (map varP fnames)

  let tdname = toDictName sname

  tdsigt <- [t| $(conT name) -> [(String, JSON.JSValue)] |]

  let felems = map (uncurry save_fn) (zip fnames fields)

      flist = listE felems

      -- and finally convert all this to a json object

      tdlist = [| concat $flist |]

      iname = mkName "i"

  tclause <- clause [pat] (normalB tdlist) []

  cclause <- [| $makeObjE . $(varE tdname) |]

  let fname = mkName ("save" ++ sname)

  sigt <- [t| $(conT name) -> JSON.JSValue |]

  return [SigD tdname tdsigt, FunD tdname [tclause],

          SigD fname sigt, ValD (VarP fname) (NormalB cclause) []]

-- | Generates the code for saving an object's field, handling the

-- various types of fields that we have.

saveObjectField :: Name -> Field -> Q Exp

saveObjectField fvar field =

  case fieldIsOptional field of

    OptionalOmitNull -> [| case $(varE fvar) of

                             Nothing -> []

                             Just v  -> [( $nameE, JSON.showJSON v )]

                         |]

    OptionalSerializeNull -> [| case $(varE fvar) of

                                  Nothing -> [( $nameE, JSON.JSNull )]

                                  Just v  -> [( $nameE, JSON.showJSON v )]

                              |]

    NotOptional ->

      case fieldShow field of

        -- Note: the order of actual:extra is important, since for

        -- some serialisation types (e.g. Luxi), we use tuples

        -- (positional info) rather than object (name info)

        Nothing -> [| [( $nameE, JSON.showJSON $fvarE)] |]

        Just fn -> [| let (actual, extra) = $fn $fvarE

                      in ($nameE, JSON.showJSON actual):extra

                    |]

  where nameE = stringE (fieldName field)

        fvarE = varE fvar

-- | Generates the showJSON clause for a given object name.

objectShowJSON :: String -> Q Dec

objectShowJSON name = do

  body <- [| JSON.showJSON . $(varE . mkName $ "save" ++ name) |]

  return $ FunD 'JSON.showJSON [Clause [] (NormalB body) []]

-- | Generates the load object functionality.

genLoadObject :: (Field -> Q (Name, Stmt))

              -> String -> [Field] -> Q (Dec, Dec)

genLoadObject load_fn sname fields = do

  let name = mkName sname

      funname = mkName $ "load" ++ sname

      arg1 = mkName $ if null fields then "_" else "v"

      objname = mkName "o"

      opid = mkName "op_id"

  st1 <- bindS (varP objname) [| liftM JSON.fromJSObject

                                 (JSON.readJSON $(varE arg1)) |]

  fbinds <- mapM load_fn fields

  let (fnames, fstmts) = unzip fbinds

  let cval = foldl (\accu fn -> AppE accu (VarE fn)) (ConE name) fnames

      retstmt = [NoBindS (AppE (VarE 'return) cval)]

      -- FIXME: should we require an empty dict for an empty type?

      -- this allows any JSValue right now

      fstmts' = if null fields

                  then retstmt

                  else st1:fstmts ++ retstmt

  sigt <- [t| JSON.JSValue -> JSON.Result $(conT name) |]

  return $ (SigD funname sigt,

            FunD funname [Clause [VarP arg1] (NormalB (DoE fstmts')) []])

-- | Generates code for loading an object's field.

loadObjectField :: Field -> Q (Name, Stmt)

loadObjectField field = do

  let name = fieldVariable field

  fvar <- newName name

  -- these are used in all patterns below

  let objvar = varNameE "o"

      objfield = stringE (fieldName field)

      loadexp =

        if fieldIsOptional field /= NotOptional

          -- we treat both optional types the same, since

          -- 'maybeFromObj' can deal with both missing and null values

          -- appropriately (the same)

          then [| $(varE 'maybeFromObj) $objvar $objfield |]

          else case fieldDefault field of

                 Just defv ->

                   [| $(varE 'fromObjWithDefault) $objvar

                      $objfield $defv |]

                 Nothing -> [| $fromObjE $objvar $objfield |]

  bexp <- loadFn field loadexp objvar

  return (fvar, BindS (VarP fvar) bexp)

-- | Builds the readJSON instance for a given object name.

objectReadJSON :: String -> Q Dec

objectReadJSON name = do

  let s = mkName "s"

  body <- [| case JSON.readJSON $(varE s) of

               JSON.Ok s' -> $(varE .mkName $ "load" ++ name) s'

               JSON.Error e ->

                 JSON.Error $ "Can't parse value for type " ++

                       $(stringE name) ++ ": " ++ e

           |]

  return $ FunD 'JSON.readJSON [Clause [VarP s] (NormalB body) []]

-- * Inheritable parameter tables implementation

-- | Compute parameter type names.

paramTypeNames :: String -> (String, String)

paramTypeNames root = ("Filled"  ++ root ++ "Params",

                       "Partial" ++ root ++ "Params")

-- | Compute information about the type of a parameter field.

paramFieldTypeInfo :: String -> Field -> Q (Name, Strict, Type)

paramFieldTypeInfo field_pfx fd = do

  t <- actualFieldType fd

  let n = mkName . (++ "P") . (field_pfx ++) .

          fieldRecordName $ fd

  return (n, NotStrict, AppT (ConT ''Maybe) t)

-- | Build a parameter declaration.

--

-- This function builds two different data structures: a /filled/ one,

-- in which all fields are required, and a /partial/ one, in which all

-- fields are optional. Due to the current record syntax issues, the

-- fields need to be named differrently for the two structures, so the

-- partial ones get a /P/ suffix.

buildParam :: String -> String -> [Field] -> Q [Dec]

buildParam sname field_pfx fields = do

  let (sname_f, sname_p) = paramTypeNames sname

      name_f = mkName sname_f

      name_p = mkName sname_p

  fields_f <- mapM (fieldTypeInfo field_pfx) fields

  fields_p <- mapM (paramFieldTypeInfo field_pfx) fields

  let decl_f = RecC name_f fields_f

      decl_p = RecC name_p fields_p

  let declF = DataD [] name_f [] [decl_f] [''Show, ''Eq]

      declP = DataD [] name_p [] [decl_p] [''Show, ''Eq]

  ser_decls_f <- buildObjectSerialisation sname_f fields

  ser_decls_p <- buildPParamSerialisation sname_p fields

  fill_decls <- fillParam sname field_pfx fields

  return $ [declF, declP] ++ ser_decls_f ++ ser_decls_p ++ fill_decls ++

           buildParamAllFields sname fields ++

           buildDictObjectInst name_f sname_f

-- | Builds a list of all fields of a parameter.

buildParamAllFields :: String -> [Field] -> [Dec]

buildParamAllFields sname fields =

  let vname = mkName ("all" ++ sname ++ "ParamFields")

      sig = SigD vname (AppT ListT (ConT ''String))

      val = ListE $ map (LitE . StringL . fieldName) fields

  in [sig, ValD (VarP vname) (NormalB val) []]

-- | Builds the 'DictObject' instance for a filled parameter.

buildDictObjectInst :: Name -> String -> [Dec]

buildDictObjectInst name sname =

  [InstanceD [] (AppT (ConT ''DictObject) (ConT name))

   [ValD (VarP 'toDict) (NormalB (VarE (toDictName sname))) []]]

-- | Generates the serialisation for a partial parameter.

buildPParamSerialisation :: String -> [Field] -> Q [Dec]

buildPParamSerialisation sname fields = do

  let name = mkName sname

  savedecls <- genSaveObject savePParamField sname fields

  (loadsig, loadfn) <- genLoadObject loadPParamField sname fields

  shjson <- objectShowJSON sname

  rdjson <- objectReadJSON sname

  let instdecl = InstanceD [] (AppT (ConT ''JSON.JSON) (ConT name))

                 [rdjson, shjson]

  return $ savedecls ++ [loadsig, loadfn, instdecl]

-- | Generates code to save an optional parameter field.

savePParamField :: Name -> Field -> Q Exp

savePParamField fvar field = do

  checkNonOptDef field

  let actualVal = mkName "v"

  normalexpr <- saveObjectField actualVal field

  -- we have to construct the block here manually, because we can't

  -- splice-in-splice

  return $ CaseE (VarE fvar) [ Match (ConP 'Nothing [])

                                       (NormalB (ConE '[])) []

                             , Match (ConP 'Just [VarP actualVal])

                                       (NormalB normalexpr) []

                             ]

-- | Generates code to load an optional parameter field.

loadPParamField :: Field -> Q (Name, Stmt)

loadPParamField field = do

  checkNonOptDef field

  let name = fieldName field

  fvar <- newName name

  -- these are used in all patterns below

  let objvar = varNameE "o"

      objfield = stringE name

      loadexp = [| $(varE 'maybeFromObj) $objvar $objfield |]

  bexp <- loadFn field loadexp objvar

  return (fvar, BindS (VarP fvar) bexp)

-- | Builds a simple declaration of type @n_x = fromMaybe f_x p_x@.

buildFromMaybe :: String -> Q Dec

buildFromMaybe fname =

  valD (varP (mkName $ "n_" ++ fname))

         (normalB [| $(varE 'fromMaybe)

                        $(varNameE $ "f_" ++ fname)

                        $(varNameE $ "p_" ++ fname) |]) []

-- | Builds a function that executes the filling of partial parameter

-- from a full copy (similar to Python's fillDict).

fillParam :: String -> String -> [Field] -> Q [Dec]

fillParam sname field_pfx fields = do

  let fnames = map (\fd -> field_pfx ++ fieldRecordName fd) fields

      (sname_f, sname_p) = paramTypeNames sname

      oname_f = "fobj"

      oname_p = "pobj"

      name_f = mkName sname_f

      name_p = mkName sname_p

      fun_name = mkName $ "fill" ++ sname ++ "Params"

      le_full = ValD (ConP name_f (map (VarP . mkName . ("f_" ++)) fnames))

                (NormalB . VarE . mkName $ oname_f) []

      le_part = ValD (ConP name_p (map (VarP . mkName . ("p_" ++)) fnames))

                (NormalB . VarE . mkName $ oname_p) []

      obj_new = foldl (\accu vname -> AppE accu (VarE vname)) (ConE name_f)

                $ map (mkName . ("n_" ++)) fnames

  le_new <- mapM buildFromMaybe fnames

  funt <- [t| $(conT name_f) -> $(conT name_p) -> $(conT name_f) |]

  let sig = SigD fun_name funt

      fclause = Clause [VarP (mkName oname_f), VarP (mkName oname_p)]

                (NormalB $ LetE (le_full:le_part:le_new) obj_new) []

      fun = FunD fun_name [fclause]

  return [sig, fun]

-- * Template code for exceptions

-- | Exception simple error message field.

excErrMsg :: (String, Q Type)

excErrMsg = ("errMsg", [t| String |])

-- | Builds an exception type definition.

genException :: String                  -- ^ Name of new type

             -> SimpleObject -- ^ Constructor name and parameters

             -> Q [Dec]

genException name cons = do

  let tname = mkName name

  declD <- buildSimpleCons tname cons

  (savesig, savefn) <- genSaveSimpleObj tname ("save" ++ name) cons $

                         uncurry saveExcCons

  (loadsig, loadfn) <- genLoadExc tname ("load" ++ name) cons

  return [declD, loadsig, loadfn, savesig, savefn]

-- | Generates the \"save\" clause for an entire exception constructor.

--

-- This matches the exception with variables named the same as the

-- constructor fields (just so that the spliced in code looks nicer),

-- and calls showJSON on it.

saveExcCons :: String        -- ^ The constructor name

            -> [SimpleField] -- ^ The parameter definitions for this

                             -- constructor

            -> Q Clause      -- ^ Resulting clause

saveExcCons sname fields = do

  let cname = mkName sname

  fnames <- mapM (newName . fst) fields

  let pat = conP cname (map varP fnames)

      felems = if null fnames

                 then conE '() -- otherwise, empty list has no type

                 else listE $ map (\f -> [| JSON.showJSON $(varE f) |]) fnames

  let tup = tupE [ litE (stringL sname), felems ]

  clause [pat] (normalB [| JSON.showJSON $tup |]) []

-- | Generates load code for a single constructor of an exception.

--

-- Generates the code (if there's only one argument, we will use a

-- list, not a tuple:

--

-- @

-- do

--  (x1, x2, ...) <- readJSON args

--  return $ Cons x1 x2 ...

-- @

loadExcConstructor :: Name -> String -> [SimpleField] -> Q Exp

loadExcConstructor inname sname fields = do

  let name = mkName sname

  f_names <- mapM (newName . fst) fields

  let read_args = AppE (VarE 'JSON.readJSON) (VarE inname)

  let binds = case f_names of

                [x] -> BindS (ListP [VarP x])

                _   -> BindS (TupP (map VarP f_names))

      cval = foldl (\accu fn -> AppE accu (VarE fn)) (ConE name) f_names

  return $ DoE [binds read_args, NoBindS (AppE (VarE 'return) cval)]

{-| Generates the loadException function.

This generates a quite complicated function, along the lines of:

@

loadFn (JSArray [JSString name, args]) = case name of

   "A1" -> do

     (x1, x2, ...) <- readJSON args

     return $ A1 x1 x2 ...

   "a2" -> ...

   s -> fail $ "Unknown exception" ++ s

loadFn v = fail $ "Expected array but got " ++ show v

@

-}

genLoadExc :: Name -> String -> SimpleObject -> Q (Dec, Dec)

genLoadExc tname sname opdefs = do

  let fname = mkName sname

  exc_name <- newName "name"

  exc_args <- newName "args"

  exc_else <- newName "s"

  arg_else <- newName "v"

  fails <- [| fail $ "Unknown exception '" ++ $(varE exc_else) ++ "'" |]

  -- default match for unknown exception name

  let defmatch = Match (VarP exc_else) (NormalB fails) []

  -- the match results (per-constructor blocks)

  str_matches <-

    mapM (\(s, params) -> do

            body_exp <- loadExcConstructor exc_args s params

            return $ Match (LitP (StringL s)) (NormalB body_exp) [])

    opdefs

  -- the first function clause; we can't use [| |] due to TH

  -- limitations, so we have to build the AST by hand

  let clause1 = Clause [ConP 'JSON.JSArray

                               [ListP [ConP 'JSON.JSString [VarP exc_name],

                                            VarP exc_args]]]

                (NormalB (CaseE (AppE (VarE 'JSON.fromJSString)

                                        (VarE exc_name))

                          (str_matches ++ [defmatch]))) []

  -- the fail expression for the second function clause

  fail_type <- [| fail $ "Invalid exception: expected '(string, [args])' " ++

                  "      but got " ++ show (pp_value $(varE arg_else)) ++ "'"

                |]

  -- the second function clause

  let clause2 = Clause [VarP arg_else] (NormalB fail_type) []

  sigt <- [t| JSON.JSValue -> JSON.Result $(conT tname) |]

  return $ (SigD fname sigt, FunD fname [clause1, clause2])