Synnefo » snf-ganeti

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}

{-| 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 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

                  , makeJSONInstance

                  ) where

import Data.Char

import Language.Haskell.TH

import qualified Text.JSON as JSON

-- | Ensure first letter is lowercase.

--

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

-- aaToString@.

ensureLower :: String -> String

ensureLower [] = []

ensureLower (x:xs) = toLower x:xs

-- | ToString function name.

toStrName :: String -> Name

toStrName = mkName . (++ "ToString") . ensureLower

-- | FromString function name.

fromStrName :: String -> Name

fromStrName = mkName . (++ "FromString") . ensureLower

-- | 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 toString function.

--

-- This generates a simple function of the form:

--

-- @

-- nameToString :: Name -> String

-- nameToString Cons1 = var1

-- nameToString Cons2 = \"value2\"

-- @

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

genToString fname tname constructors = do

  sigt <- [t| $(conT tname) -> String |]

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

  -- string value

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

                             (normalB (varE  v)) []) constructors

  return [SigD fname sigt, FunD fname clauses]

-- | Generates a fromString function.

--

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

-- string. It is of the form:

--

-- @

-- nameFromString :: (Monad m) => String -> m Name

-- nameFromString s | s == var1       = Cons1

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

--                  | otherwise = fail /.../

-- @

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

genFromString fname tname constructors = do

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

  sigt <- [t| (Monad m) => String -> 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))) ++ ": " ++ $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 string 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/ToString, which converts the type to a string

--

-- * /name/FromString, which (monadically) converts from a string to the type

--

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

-- nothing else.

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

declareSADT sname cons = do

  let name = mkName sname

      ddecl = strADTDecl name (map fst cons)

  tostr <- genToString (toStrName sname) name cons

  fromstr <- genFromString (fromStrName sname) name cons

  return $ ddecl:tostr ++ fromstr

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

--

-- This will create what is the equivalent of:

--

-- @

-- showJSON = showJSON . /name/ToString

-- @

--

-- in an instance JSON /name/ declaration

genShowJSON :: String -> Q [Dec]

genShowJSON name = [d| showJSON = JSON.showJSON . $(varE (toStrName name)) |]

-- | 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/FromString 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 (fromStrName name)) s'

               JSON.Error e ->

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

                           $(litE (StringL name)) ++ ": " ++ e

           |]

  return $ FunD (mkName "readJSON") [Clause [VarP s] (NormalB body) []]

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

--

-- This assumes that the /name/ToString and /name/FromString 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)]