{-# LANGUAGE TemplateHaskell #-}
-{-| TemplateHaskell helper for HTools.
+{-| 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
, declareIADT
, makeJSONInstance
, genOpID
+ , genAllOpIDs
, genOpCode
, genStrOfOp
, genStrOfKey
, defaultField
, optionalField
, renameField
- , containerField
, customField
, timeStampFields
, uuidFields
, serialFields
+ , tagsFields
+ , TagSet
, buildObject
, buildObjectSerialisation
, buildParam
- , Container
+ , DictObject(..)
+ , genException
+ , excErrMsg
) where
-import Control.Arrow
-import Control.Monad (liftM, liftM2)
+import Control.Monad (liftM)
import Data.Char
import Data.List
-import qualified Data.Map as M
+import Data.Maybe (fromMaybe)
+import qualified Data.Set as Set
import Language.Haskell.TH
import qualified Text.JSON as JSON
-
-import Ganeti.HTools.JSON
+import Text.JSON.Pretty (pp_value)
-- * Exported types
-type Container = M.Map String
+-- | Class of objects that can be converted to 'JSObject'
+-- lists-format.
+class DictObject a where
+ toDict :: a -> [(String, JSON.JSValue)]
-- | Serialised field data type.
data Field = Field { fieldName :: String
, fieldShow :: Maybe (Q Exp)
, fieldDefault :: Maybe (Q Exp)
, fieldConstr :: Maybe String
- , fieldIsContainer :: Bool
, fieldIsOptional :: Bool
}
, fieldShow = Nothing
, fieldDefault = Nothing
, fieldConstr = Nothing
- , fieldIsContainer = False
, fieldIsOptional = False
}
optionalField :: Field -> Field
optionalField field = field { fieldIsOptional = True }
--- | Marks a field as a container.
-containerField :: Field -> Field
-containerField field = field { fieldIsContainer = True }
-
-- | Sets custom functions on a field.
-customField :: Q Exp -> Q Exp -> Field -> 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 readfn, fieldShow = Just showfn }
+ field { fieldRead = Just (varE readfn), fieldShow = Just (varE showfn) }
+-- | 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 }) =
- maybe (camelCase name) id 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
fieldVariable f =
case (fieldConstr f) of
Just name -> ensureLower name
- _ -> fieldName f
+ _ -> map (\c -> if c == '-' then '_' else c) $ fieldName f
actualFieldType :: Field -> Q Type
-actualFieldType f | fieldIsContainer f = [t| Container $t |]
- | fieldIsOptional f = [t| Maybe $t |]
+actualFieldType f | fieldIsOptional f = [t| Maybe $t |]
| otherwise = t
where t = fieldType f
fail $ "Default field " ++ name ++ " used in parameter declaration"
checkNonOptDef _ = return ()
-loadFn :: Field -> Q Exp -> Q Exp
-loadFn (Field { fieldIsContainer = True }) expr = [| $expr >>= readContainer |]
-loadFn (Field { fieldRead = Just readfn }) expr = [| $expr >>= $readfn |]
-loadFn _ expr = expr
-
+-- | 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]
+
-- * Helper functions
-- | Ensure first letter is lowercase.
fromRawName :: String -> Name
fromRawName = mkName . (++ "FromRaw") . ensureLower
--- | Converts a name to it's varE/litE representations.
---
+-- | Converts a name to it's varE\/litE representations.
reprE :: Either String Name -> Q Exp
reprE = either stringE varE
appFn f x | f == VarE 'id = x
| otherwise = AppE f x
--- | Container loader
-readContainer :: (Monad m, JSON.JSON a) =>
- JSON.JSObject JSON.JSValue -> m (Container a)
-readContainer obj = do
- let kjvlist = JSON.fromJSObject obj
- kalist <- mapM (\(k, v) -> fromKeyValue k v >>= \a -> return (k, a)) kjvlist
- return $ M.fromList kalist
+-- | Builds a field for a normal constructor.
+buildConsField :: Q Type -> StrictTypeQ
+buildConsField ftype = do
+ ftype' <- ftype
+ return (NotStrict, ftype')
--- | Container dumper
-showContainer :: (JSON.JSON a) => Container a -> JSON.JSValue
-showContainer = JSON.makeObj . map (second JSON.showJSON) . M.toList
+-- | 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, ''Read, ''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
--
-- * /name/FromRaw, which (monadically) converts from a raw type to the type
--
--- Note that this is basically just a custom show/read instance,
+-- 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
-- | Transform an underscore_name into a CamelCase one.
camelCase :: String -> String
camelCase = concatMap (ensureUpper . drop 1) .
- groupBy (\_ b -> b /= '_') . ('_':)
+ groupBy (\_ b -> b /= '_' && b /= '-') . ('_':)
-- | Computes the name of a given constructor.
constructorName :: Con -> Q 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 (ConStructorTwo {}) = trans_fun("ConStructorTwo")
-- @
--
--- This builds a custom list of name/string pairs and then uses
--- 'genToRaw' to actually generate the function
+-- 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
+ cnames <- reifyConsNames name
let svalues = map (Left . trans_fun) cnames
genToRaw ''String (mkName fname) name $ zip cnames svalues
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)
-> Q Clause -- ^ Resulting clause
saveConstructor sname fields = do
let cname = mkName sname
- let fnames = map (mkName . fieldVariable) fields
+ 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
sigt <- [t| $(conT (mkName "OpCode")) -> JSON.JSValue |]
return $ (SigD fname sigt, FunD fname cclauses)
+-- | Generates load code for a single constructor of the opcode data type.
loadConstructor :: String -> [Field] -> Q Exp
loadConstructor sname fields = do
let name = mkName sname
fstmts' = fstmts ++ [NoBindS (AppE (VarE 'return) cval)]
return $ DoE fstmts'
+-- | Generates the loadOpCode function.
genLoadOpCode :: [(String, [Field])] -> Q (Dec, Dec)
genLoadOpCode opdefs = do
let fname = mkName "loadOpCode"
genStrOfKey :: Name -> String -> Q [Dec]
genStrOfKey = genConstrToStr ensureLower
--- | LuxiOp parameter type.
-type LuxiParam = (String, Q Type, Q Exp)
-
-- | Generates the LuxiOp data type.
--
-- This takes a Luxi operation definition and builds both the
-- We can't use anything less generic, because the way different
-- operations are serialized differs on both parameter- and top-level.
--
--- There are three things to be defined for each parameter:
+-- There are two things to be defined for each parameter:
--
-- * name
--
-- * type
--
--- * operation; this is the operation performed on the parameter before
--- serialization
---
-genLuxiOp :: String -> [(String, [LuxiParam])] -> Q [Dec]
+genLuxiOp :: String -> SimpleObject -> 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
+ let tname = mkName name
+ declD <- buildSimpleCons tname cons
+ (savesig, savefn) <- genSaveSimpleObj tname "opToArgs"
+ cons saveLuxiConstructor
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, fn) =
- [| JSON.showJSON ( $(liftM2 appFn fn $ varE fvar) ) |]
+saveLuxiField :: Name -> SimpleField -> Q Exp
+saveLuxiField fvar (_, qt) =
+ [| JSON.showJSON $(varE fvar) |]
-- | Generates the \"save\" clause for entire LuxiOp constructor.
-saveLuxiConstructor :: (String, [LuxiParam]) -> Q Clause
+saveLuxiConstructor :: SimpleConstructor -> Q Clause
saveLuxiConstructor (sname, fields) = do
let cname = mkName sname
- fnames = map (\(nm, _, _) -> mkName nm) fields
+ fnames = map (mkName . fst) fields
pat = conP cname (map varP fnames)
flist = map (uncurry saveLuxiField) (zip fnames fields)
finval = if null flist
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.
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
[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
- let fnames = map (mkName . fieldVariable) fields
+ fnames <- mapM (newName . fieldVariable) fields
let pat = conP name (map varP fnames)
- let tdname = mkName ("toDict" ++ sname)
+ let tdname = toDictName sname
tdsigt <- [t| $(conT name) -> [(String, JSON.JSValue)] |]
let felems = map (uncurry save_fn) (zip fnames fields)
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
- | isContainer = [| [( $nameE , JSON.showJSON . showContainer $ $fvarE)] |]
| fisOptional = [| case $(varE fvar) of
Nothing -> []
Just v -> [( $nameE, JSON.showJSON v)]
|]
| otherwise = case fieldShow field of
Nothing -> [| [( $nameE, JSON.showJSON $fvarE)] |]
- Just fn -> [| [( $nameE, JSON.showJSON . $fn $ $fvarE)] |]
- where isContainer = fieldIsContainer field
- fisOptional = fieldIsOptional field
+ Just fn -> [| let (actual, extra) = $fn $fvarE
+ in extra ++ [( $nameE, JSON.showJSON actual)]
+ |]
+ where fisOptional = fieldIsOptional field
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 (mkName "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
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 = mkName name
+ fvar <- newName name
-- these are used in all patterns below
let objvar = varNameE "o"
objfield = stringE (fieldName field)
[| $(varNameE "fromObjWithDefault") $objvar
$objfield $defv |]
Nothing -> [| $(varNameE "fromObj") $objvar $objfield |]
- bexp <- loadFn field loadexp
+ 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"
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
-
+ 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
[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
, 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 = mkName name
+ 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
+ bexp <- loadFn field loadexp objvar
return (fvar, BindS (VarP fvar) bexp)
-- | Builds a simple declaration of type @n_x = fromMaybe f_x p_x@.
$(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
(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])