Portability | non-portable (uses GHC extensions) |
---|---|
Maintainer | lemmih@gmail.com |
SafeCopy extends the parsing and serialization capabilities of Data.Binary to include nested version control. Nested version control means that you can change the defintion 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.Binary together with Haskell's easy of use. However, things quickly takes a 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
- class SafeCopy (MigrateFrom a) => Migrate a where
- type MigrateFrom a
- migrate :: MigrateFrom a -> a
- data Kind a
- extension :: (SafeCopy a, Migrate a) => Kind a
- data Contained a
- contain :: a -> Contained a
- data Version a
- getSafeGet :: SafeCopy a => Get (Get a)
- getSafePut :: 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 through getCopy
and putCopy
can't be used directly.
To serialize/parse a data type using SafeCopy
, see safeGet
and safePut
.
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.
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.
type MigrateFrom a Source
This is the type we're extending. Each type capable of migration can only extend one other type.
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.
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.
A simple numeric version id.
Rarely used functions
getSafeGet :: 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 :: 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
.