Synnefo » snf-ganeti

{-# LANGUAGE TemplateHaskell #-}

{-| TemplateHaskell helper for HTools.

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

                  , declareIADT

                  , makeJSONInstance

                  , genOpID

                  , genOpCode

                  , genStrOfOp

                  , genStrOfKey

                  , genLuxiOp

                  , Field

                  , simpleField

                  , defaultField

                  , optionalField

                  , renameField

                  , customField

                  , timeStampFields

                  , uuidFields

                  , serialFields

                  , tagsFields

                  , buildObject

                  , buildObjectSerialisation

                  , buildParam

                  ) 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

-- * Exported types

-- | Serialised field data type.

data Field = Field { fieldName        :: String

                   , fieldType        :: Q Type

                   , fieldRead        :: Maybe (Q Exp)

                   , fieldShow        :: Maybe (Q Exp)

                   , fieldDefault     :: Maybe (Q Exp)

                   , fieldConstr      :: Maybe String

                   , fieldIsOptional  :: Bool

                   }

-- | Generates a simple field.

simpleField :: String -> Q Type -> Field

simpleField fname ftype =

  Field { fieldName        = fname

        , fieldType        = ftype

        , fieldRead        = Nothing

        , fieldShow        = Nothing

        , fieldDefault     = Nothing

        , fieldConstr      = Nothing

        , fieldIsOptional  = False

        }

-- | 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 = True }

-- | Sets custom functions on a field.

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

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

            -> Field   -- ^ The original field

            -> Field   -- ^ Updated field

customField readfn showfn field =

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

fieldRecordName :: Field -> String

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

  maybe (camelCase name) id 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

actualFieldType :: Field -> Q Type

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

                  | otherwise = t

                  where t = fieldType f

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

checkNonOptDef (Field { fieldIsOptional = True, 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

timeStampFields :: [Field]

timeStampFields =

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

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

    ]

serialFields :: [Field]

serialFields =

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

uuidFields :: [Field]

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

-- | Tag field description.

tagsFields :: [Field]

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

               simpleField "tags" [t| Set.Set String |] ]

-- * 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 = varNameE "showJSON"

-- | 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

-- * 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, ''Read, ''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, 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 == $(varE 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 :: Name -> String -> [(String, Name)] -> Q [Dec]

declareADT 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, Right b)) cons

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

  fromraw <- genFromRaw traw (fromRawName sname) name cons

  return $ ddecl:toraw ++ fromraw

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

declareIADT = declareADT ''Int

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

declareSADT = declareADT ''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 (mkName "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 (mkName "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

-- | 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

  TyConI (DataD _ _ _ cons _) <- reify name

  cnames <- mapM (liftM nameBase . constructorName) cons

  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

-- | OpCode parameter (field) type.

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

-- | 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

          -> [(String, [Field])]   -- ^ Constructor name and parameters

          -> Q [Dec]

genOpCode name cons = do

  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, ''Read, ''Eq]

  (savesig, savefn) <- genSaveOpCode cons

  (loadsig, loadfn) <- genLoadOpCode cons

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

-- | Checks whether a given parameter is options.

--

-- This requires that it's a 'Maybe'.

isOptional :: Type -> Bool

isOptional (AppT (ConT dt) _) | dt == ''Maybe = True

isOptional _ = False

-- | 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 :: String    -- ^ The constructor name

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

                             -- 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' = [| $(varNameE "makeObj") (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 :: [(String, [Field])] -> Q (Dec, Dec)

genSaveOpCode opdefs = do

  cclauses <- mapM (uncurry saveConstructor) opdefs

  let fname = mkName "saveOpCode"

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

  return $ (SigD fname sigt, FunD fname cclauses)

loadConstructor :: String -> [Field] -> 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'

genLoadOpCode :: [(String, [Field])] -> 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) [| $(varNameE "fromObj")

                              $(varE objname) $(stringE "OP_ID") |]

  -- the match results (per-constructor blocks)

  mexps <- mapM (uncurry loadConstructor) opdefs

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

  let mpats = map (\(me, c) ->

                       let mp = LitP . StringL . deCamelCase . fst $ c

                       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

-- | LuxiOp parameter type.

type LuxiParam = (String, Q Type)

-- | Generates the LuxiOp data type.

--

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

-- datatype and the function trnasforming 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 -> [(String, [LuxiParam])] -> Q [Dec]

genLuxiOp name cons = do

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

                    fields' <- mapM (\(_, qt) ->

                                         qt >>= \t -> return (NotStrict, t))

                               fields

                    return $ NormalC (mkName cname) fields')

            cons

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

  (savesig, savefn) <- genSaveLuxiOp cons

  req_defs <- declareSADT "LuxiReq" .

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

                  cons

  return $ [declD, savesig, savefn] ++ req_defs

-- | Generates the \"save\" expression for a single luxi parameter.

saveLuxiField :: Name -> LuxiParam -> Q Exp

saveLuxiField fvar (_, qt) =

    [| JSON.showJSON $(varE fvar) |]

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

saveLuxiConstructor :: (String, [LuxiParam]) -> Q Clause

saveLuxiConstructor (sname, fields) = do

  let cname = mkName sname

      fnames = map (mkName . fst) fields

      pat = conP cname (map varP fnames)

      flist = map (uncurry saveLuxiField) (zip fnames fields)

      finval = if null flist

               then [| JSON.showJSON ()    |]

               else [| JSON.showJSON $(listE flist) |]

  clause [pat] (normalB finval) []

-- | Generates the main save LuxiOp function.

genSaveLuxiOp :: [(String, [LuxiParam])]-> Q (Dec, Dec)

genSaveLuxiOp opdefs = do

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

  let fname = mkName "opToArgs"

  cclauses <- mapM saveLuxiConstructor opdefs

  return $ (SigD fname sigt, FunD fname cclauses)

-- * "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, ''Read, ''Eq]

  ser_decls <- buildObjectSerialisation sname fields

  return $ declD:ser_decls

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]

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 = mkName ("toDict" ++ 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 <- [| $(varNameE "makeObj") . $(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) []]

saveObjectField :: Name -> Field -> Q Exp

saveObjectField fvar field

  | fisOptional = [| case $(varE fvar) of

                      Nothing -> []

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

                  |]

  | otherwise = case fieldShow field of

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

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

                    in extra ++ [( $nameE, JSON.showJSON actual)]

                  |]

  where fisOptional  = fieldIsOptional field

        nameE = stringE (fieldName field)

        fvarE = varE fvar

objectShowJSON :: String -> Q Dec

objectShowJSON name = do

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

  return $ FunD (mkName "showJSON") [Clause [] (NormalB body) []]

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 "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

      fstmts' = st1:fstmts ++ [NoBindS (AppE (VarE 'return) cval)]

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

  return $ (SigD funname sigt,

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

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

          then [| $(varNameE "maybeFromObj") $objvar $objfield |]

          else case fieldDefault field of

                 Just defv ->

                   [| $(varNameE "fromObjWithDefault") $objvar

                      $objfield $defv |]

                 Nothing -> [| $(varNameE "fromObj") $objvar $objfield |]

  bexp <- loadFn field loadexp objvar

  return (fvar, BindS (VarP fvar) bexp)

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 (mkName "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, ''Read, ''Eq]

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

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]

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) []

                             ]

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 = [| $(varNameE "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 [| $(varNameE "fromMaybe")

                        $(varNameE $ "f_" ++ fname)

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

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]