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A library to support global entities in Curry programs.
A global entity has a name declared as a top-level entity.
Its value can be accessed and modified by IO actions.
Global entities can be declared as persistent so that
their values are stored across different program executions
or temporary so that they will be stored only in memory.
Currently, it is still experimental so that its interface might
be slightly changed in the future.
A temporary global entity `gt` is a top-level constant of type
`GlobalT t`. If `v` is an initial value `v` of type `t`,
where the type `t` does not contain type variables or type class
contraints, the temporary global entity should be declared in
a module `Mod` by:
gt :: GlobalT t
gt = globalT "Mod.gt" v
The first argument is the qualified name of this program entity
and used as a unique name for this global value.
Similarly, a persistent global entity `gp` with an initial value `v`
of type `t` could be declared by:
gt :: GlobalP t
gt = globalPersistent f v
where the type `t` must not contain type variables and support
`Read` and `Show` instances. `f` is the file name
where the global value is persistently stored
(the file is created and initialized with `v` if it does not exist).
@author Michael Hanus
@version December 2023
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sourcecode:
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{-# LANGUAGE CPP #-}
module Data.Global
( GlobalT, globalT, readGlobalT, writeGlobalT
, GlobalP, globalP, globalPersistent
, readGlobalP, safeReadGlobalP, writeGlobalP
) where
import Control.Monad ( unless )
import System.Directory ( doesFileExist )
import System.IO
import System.IOExts ( exclusiveIO )
import System.IO.Unsafe ( unsafePerformIO )
------------------------------------------------------------------------------
-- Implementation of temporary global entities.
-- The implementation requires a specific compiler feature
-- to to translate top-level entities of type `Data.Global.GlobalT`
-- in a specific way, i.e., as constants rather than operations.
--- The abstract type of a temporary global entity.
#ifdef __KICS2__
external data GlobalT _
#else
data GlobalT _ = GlobalT String
#endif
--- `globalT` is used only to declare a temporary global value
--- as a top-level entity. It should not be used elsewhere.
--- The first argument is the unique name of the temporary global entity.
--- It should be the qualified name of the corresponding program entity.
--- The second argument is the initial value which will be evaluated
--- to a ground normal form when the global entity is used for the first time.
globalT :: String -> a -> GlobalT a
globalT n v = (prim_globalT $## n) v
prim_globalT :: String -> a -> GlobalT a
prim_globalT external
--- Reads the current value of a temporary global entity.
readGlobalT :: GlobalT a -> IO a
readGlobalT g = prim_readGlobalT $# g
prim_readGlobalT :: GlobalT a -> IO a
prim_readGlobalT external
--- Updates the value of a temporary global entity.
--- The new value is evaluated to a ground normal form before updating
--- the entity.
writeGlobalT :: GlobalT a -> a -> IO ()
writeGlobalT g v = (prim_writeGlobalT $# g) $## v
prim_writeGlobalT :: GlobalT a -> a -> IO ()
prim_writeGlobalT external
------------------------------------------------------------------------------
-- Implementation of persistent global entities.
--- The abstract type of a persistent global entity.
data GlobalP _ = GlobalP String
--- `globalP` is used only to declare a persistent global value
--- as a top-level entity. It should not be used elsewhere.
--- The first argument is the file name where the global value
--- is persistently stored. The file is created and initialized
--- with the second argument if it does not exist.
globalP :: (Read a, Show a) => String -> a -> GlobalP a
globalP f v = unsafePerformIO $ do
exf <- doesFileExist f
unless exf $ writeGlobalP (GlobalP f) v
return $ GlobalP f
--- `globalPersistent` is used only to declare a persistent global value
--- as a top-level entity. It should not be used elsewhere.
--- The first argument is the file name where the global value
--- is persistently stored. The file is created and initialized
--- with the second argument if it does not exist.
globalPersistent :: (Read a, Show a) => String -> a -> GlobalP a
globalPersistent = globalP
--- Reads the current value of a persistent global entity.
readGlobalP :: Read a => GlobalP a -> IO a
readGlobalP (GlobalP f) = exclusiveIO (f ++ ".LOCK") $ do
cnt <- openFile f ReadMode >>= hGetContents
case reads cnt of
[(x,_)] -> return x
_ -> error $ "Cannot read persistent global entity in file: " ++ f ++
"\nFile contents:\n" ++ cnt
--- Safely reads the current value of a global.
--- If `readGlobalP` fails (e.g., due to a corrupted persistent storage),
--- the global is re-initialized with the default value given as
--- the second argument.
safeReadGlobalP :: (Read a, Show a) => GlobalP a -> a -> IO a
safeReadGlobalP g dflt =
catch (readGlobalP g)
(\err -> do hPutStrLn stderr $ "Error caught in 'safeReadGlobalP':\n" ++
show err
writeGlobalP g dflt >> return dflt )
--- Updates the value of a persistent global entity.
writeGlobalP :: Show a => GlobalP a -> a -> IO ()
writeGlobalP (GlobalP f) v =
exclusiveIO (f ++ ".LOCK") $ writeFile f (show v ++ "\n")
------------------------------------------------------------------------
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