| Portability | non-portable (uses GHC extensions) |
|---|---|
| Maintainer | lemmih@gmail.com |
| Safe Haskell | None |
Data.SafeCopy
Description
SafeCopy extends the parsing and serialization capabilities of Data.Serialize to include nested version control. Nested version control means that you can change the definition and binary format of a type nested deep within other types without problems.
Consider this scenario. You want to store your contact list on disk and so write the following code:
type Name = String
type Address = String
data Contacts = Contacts [(Name, Address)]
instance SafeCopy Contacts where
putCopy (Contacts list) = contain $ safePut list
getCopy = contain $ Contacts <$> safeGet
At this point, everything is fine. You get the awesome speed of Data.Serialize together with Haskell's ease of use. However, things quickly take a U-turn for the worse when you realize that you want to keep phone numbers as well as names and addresses. Being the experienced coder that you are, you see that using a 3-tuple isn't very pretty and you'd rather use a record. At first you fear that this change in structure will invalidate all your old data. Those fears are quickly quelled, though, when you remember how nifty SafeCopy is. With renewed enthusiasm, you set out and write the following code:
type Name = String
type Address = String
type Phone = String
{- We rename our old Contacts structure -}
data Contacts_v0 = Contacts_v0 [(Name, Address)]
instance SafeCopy Contacts_v0 where
putCopy (Contacts_v0 list) = contain $ safePut list
getCopy = contain $ Contacts_v0 <$> safeGet
data Contact = Contact { name :: Name
, address :: Address
, phone :: Phone }
instance SafeCopy Contact where
putCopy Contact{..} = contain $ do safePut name; safePut address; safePut phone
getCopy = contain $ Contact <$> safeGet <*> safeGet <*> safeGet
data Contacts = Contacts [Contact]
instance SafeCopy Contacts where
version = 2
kind = extension
putCopy (Contacts contacts) = contain $ safePut contacts
getCopy = contain $ Contacts <$> safeGet
{- Here the magic happens: -}
instance Migrate Contacts where
type MigrateFrom Contacts = Contacts_v0
migrate (Contacts_v0 contacts) = Contacts [ Contact{ name = name
, address = address
, phone = "" }
| (name, address) <- contacts ]
With this, you reflect on your code and you are happy. You feel confident in the safety of
your data and you know you can remove Contacts_v0 once you no longer wish to support
that legacy format.
- safeGet :: SafeCopy a => Get a
- safePut :: SafeCopy a => a -> Put
- class SafeCopy a where
- data Profile a
- newtype Prim a = Prim {
- getPrimitive :: a
- class SafeCopy (MigrateFrom a) => Migrate a where
- type MigrateFrom a
- migrate :: MigrateFrom a -> a
- newtype Reverse a = Reverse {
- unReverse :: a
- data Kind a
- extension :: (SafeCopy a, Migrate a) => Kind a
- extended_extension :: (SafeCopy a, Migrate a, Migrate (Reverse a)) => Kind a
- extended_base :: Migrate (Reverse a) => Kind a
- base :: Kind a
- data Contained a
- contain :: a -> Contained a
- data Version a
- deriveSafeCopy :: Version a -> Name -> Name -> Q [Dec]
- deriveSafeCopyIndexedType :: Version a -> Name -> Name -> [Name] -> Q [Dec]
- deriveSafeCopySimple :: Version a -> Name -> Name -> Q [Dec]
- deriveSafeCopySimpleIndexedType :: Version a -> Name -> Name -> [Name] -> Q [Dec]
- deriveSafeCopyHappstackData :: Version a -> Name -> Name -> Q [Dec]
- deriveSafeCopyHappstackDataIndexedType :: Version a -> Name -> Name -> [Name] -> Q [Dec]
- getSafeGet :: forall a. SafeCopy a => Get (Get a)
- getSafePut :: forall a. SafeCopy a => PutM (a -> Put)
- primitive :: Kind a
Documentation
safeGet :: SafeCopy a => Get aSource
Parse a version tagged data type and then migrate it to the desired type. Any serialized value has been extended by the return type can be parsed.
safePut :: SafeCopy a => a -> PutSource
Serialize a data type by first writing out its version tag. This is much
simpler than the corresponding safeGet since previous versions don't
come into play.
The centerpiece of this library. Defines a version for a data type together with how it should be serialized/parsed.
Users should define instances of SafeCopy for their types
even though getCopy and putCopy can't be used directly.
To serialize/parse a data type using SafeCopy, see safeGet
and safePut.
Methods
The version of the type.
Only used as a key so it must be unique (this is checked at run-time) but doesn't have to be sequential or continuous.
The default version is '0'.
The kind specifies how versions are dealt with. By default,
values are tagged with their version id and don't have any
previous versions. See extension and the much less used
primitive.
getCopy :: Contained (Get a)Source
This method defines how a value should be parsed without also worrying
about writing out the version tag. This function cannot be used directly.
One should use safeGet, instead.
putCopy :: a -> Contained PutSource
This method defines how a value should be parsed without worrying about
previous versions or migrations. This function cannot be used directly.
One should use safeGet, instead.
objectProfile :: Profile aSource
Version profile.
errorTypeName :: Proxy a -> StringSource
The name of the type. This is only used in error message strings. Feel free to leave undefined in your instances.
Instances
Constructors
| PrimitiveProfile | |
| InvalidProfile String | |
| Profile | |
Fields | |
Wrapper for data that was saved without a version tag.
Constructors
| Prim | |
Fields
| |
class SafeCopy (MigrateFrom a) => Migrate a whereSource
The central mechanism for dealing with version control.
This type class specifies what data migrations can happen and how they happen.
Associated Types
type MigrateFrom a Source
This is the type we're extending. Each type capable of migration can only extend one other type.
Methods
migrate :: MigrateFrom a -> aSource
This method specifies how to migrate from the older type to the newer one. It will never be necessary to use this function manually as it all taken care of internally in the library.
This is a wrapper type used migrating backwards in the chain of compatible types.
The kind of a data type determines how it is tagged (if at all).
Primitives kinds (see primitive) are not tagged with a version
id and hence cannot be extended later.
Extensions (see extension) tells the system that there exists
a previous version of the data type which should be migrated if
needed.
There is also a default kind which is neither primitive nor is an extension of a previous type.
extension :: (SafeCopy a, Migrate a) => Kind aSource
The extension kind lets the system know that there is at least one previous version of this type. A given data type can only extend a single other data type. However, it is perfectly fine to build chains of extensions. The migrations between each step is handled automatically.
extended_extension :: (SafeCopy a, Migrate a, Migrate (Reverse a)) => Kind aSource
The extended_base kind lets the system know that there is at least one future version of this type.
extended_base :: Migrate (Reverse a) => Kind aSource
The extended_base kind lets the system know that there is at least one future version of this type.
A simple numeric version id.
Template haskell functions
deriveSafeCopy :: Version a -> Name -> Name -> Q [Dec]Source
Derive an instance of SafeCopy.
When serializing, we put a Word8 describing the
constructor (if the data type has more than one
constructor). For each type used in the constructor, we
call getSafePut (which immediately serializes the version
of the type). Then, for each field in the constructor, we
use one of the put functions obtained in the last step.
For example, given the data type and the declaration below
data T0 b = T0 b Int deriveSafeCopy 1 'base ''T0
we generate
instance (SafeCopy a, SafeCopy b) =>
SafeCopy (T0 b) where
putCopy (T0 arg1 arg2) = contain $ do put_b <- getSafePut
put_Int <- getSafePut
put_b arg1
put_Int arg2
return ()
getCopy = contain $ do get_b <- getSafeGet
get_Int <- getSafeGet
return T0 <*> get_b <*> get_Int
version = 1
kind = base
And, should we create another data type as a newer version of T0, such as
data T a b = C a a | D b Int deriveSafeCopy 2 'extension ''T instance SafeCopy b => Migrate (T a b) where type MigrateFrom (T a b) = T0 b migrate (T0 b i) = D b i
we generate
instance (SafeCopy a, SafeCopy b) =>
SafeCopy (T a b) where
putCopy (C arg1 arg2) = contain $ do putWord8 0
put_a <- getSafePut
put_a arg1
put_a arg2
return ()
putCopy (D arg1 arg2) = contain $ do putWord8 1
put_b <- getSafePut
put_Int <- getSafePut
put_b arg1
put_Int arg2
return ()
getCopy = contain $ do tag <- getWord8
case tag of
0 -> do get_a <- getSafeGet
return C <*> get_a <*> get_a
1 -> do get_b <- getSafeGet
get_Int <- getSafeGet
return D <*> get_b <*> get_Int
_ -> fail $ "Could not identify tag \"" ++
show tag ++ "\" for type Main.T " ++
"that has only 2 constructors. " ++
"Maybe your data is corrupted?"
version = 2
kind = extension
Note that by using getSafePut, we saved 4 bytes in the case
of the C constructor. For D and T0, we didn't save
anything. The instance derived by this function always use
at most the same space as those generated by
deriveSafeCopySimple, but never more (as we don't call
'getSafePut'/'getSafeGet' for types that aren't needed).
Note that you may use deriveSafeCopySimple with one
version of your data type and deriveSafeCopy in another
version without any problems.
deriveSafeCopySimple :: Version a -> Name -> Name -> Q [Dec]Source
Derive an instance of SafeCopy. The instance derived by
this function is simpler than the one derived by
deriveSafeCopy in that we always use safePut and
safeGet (instead of getSafePut and getSafeGet).
When serializing, we put a Word8 describing the
constructor (if the data type has more than one constructor)
and, for each field of the constructor, we use safePut.
For example, given the data type and the declaration below
data T a b = C a a | D b Int deriveSafeCopySimple 1 'base ''T
we generate
instance (SafeCopy a, SafeCopy b) =>
SafeCopy (T a b) where
putCopy (C arg1 arg2) = contain $ do putWord8 0
safePut arg1
safePut arg2
return ()
putCopy (D arg1 arg2) = contain $ do putWord8 1
safePut arg1
safePut arg2
return ()
getCopy = contain $ do tag <- getWord8
case tag of
0 -> do return C <*> safeGet <*> safeGet
1 -> do return D <*> safeGet <*> safeGet
_ -> fail $ "Could not identify tag \"" ++
show tag ++ "\" for type Main.T " ++
"that has only 2 constructors. " ++
"Maybe your data is corrupted?"
version = 1
kind = base
Using this simpler instance means that you may spend more bytes when serializing data. On the other hand, it is more straightforward and may match any other format you used in the past.
Note that you may use deriveSafeCopy with one version of
your data type and deriveSafeCopySimple in another version
without any problems.
deriveSafeCopyHappstackData :: Version a -> Name -> Name -> Q [Dec]Source
Derive an instance of SafeCopy. The instance derived by
this function should be compatible with the instance derived
by the module Happstack.Data.SerializeTH of the
happstack-data package. The instances use only safePut
and safeGet (as do the instances created by
deriveSafeCopySimple), but we also always write a Word8
tag, even if the data type isn't a sum type.
For example, given the data type and the declaration below
data T0 b = T0 b Int deriveSafeCopy 1 'base ''T0
we generate
instance (SafeCopy a, SafeCopy b) =>
SafeCopy (T0 b) where
putCopy (T0 arg1 arg2) = contain $ do putWord8 0
safePut arg1
safePut arg2
return ()
getCopy = contain $ do tag <- getWord8
case tag of
0 -> do return T0 <*> safeGet <*> safeGet
_ -> fail $ "Could not identify tag \"" ++
show tag ++ "\" for type Main.T0 " ++
"that has only 1 constructors. " ++
"Maybe your data is corrupted?"
version = 1
kind = base
This instance always consumes at least the same space as
deriveSafeCopy or deriveSafeCopySimple, but may use more
because of the useless tag. So we recomend using it only if
you really need to read a previous version in this format,
and not for newer versions.
Note that you may use deriveSafeCopy with one version of
your data type and deriveSafeCopyHappstackData in another version
without any problems.
Rarely used functions
getSafeGet :: forall a. SafeCopy a => Get (Get a)Source
Parse a version tag and return the corresponding migrated parser. This is
useful when you can prove that multiple values have the same version.
See getSafePut.
getSafePut :: forall a. SafeCopy a => PutM (a -> Put)Source
Serialize the version tag and return the associated putter. This is useful
when serializing multiple values with the same version. See getSafeGet.