patch
Data structures for describing changes to other data structures.
| LTS Haskell 24.16: | 0.0.8.4 |
| Stackage Nightly 2025-10-24: | 0.0.8.4 |
| Latest on Hackage: | 0.0.8.4 |
patch-0.0.8.4@sha256:00904a2f2e6b9bef71b12a5200bc047d6e108af3ad74954eed1795e419949bed,3368Module documentation for 0.0.8.4
patch
Data structures for describing changes to other data structures.
A Patch type represents a kind of change made to a data structure.
class Patch p where
type PatchTarget p :: *
-- | Apply the patch p a to the value a. If no change is needed, return
-- 'Nothing'.
apply :: p -> PatchTarget p -> Maybe (PatchTarget p)
Patching Maps
For example, Data.Patch.Map defines the PatchMap type which can be used to patch Maps. A PatchMap represents updates to a Map that can insert, remove, or replace items in the Map. In this example, the Map is the PatchTarget and the PatchMap is the Patch. Keep in mind that there are many other possible Patches that can be applied to a Map (i.e., Map can be the PatchTarget for many different Patch instances).
PatchMap is defined as:
newtype PatchMap k v = PatchMap { unPatchMap :: Map k (Maybe v) }
The Maybe around the value is used to represent insertion/updates or deletion of the element at a given key.
Its Patch instance begins with:
instance Ord k => Patch (PatchMap k v) where
type PatchTarget (PatchMap k v) = Map k v
...
When a PatchMap is applied to its PatchTarget, the following changes can occur:
-
If the key is present in the
Patchand thePatchTarget…-
And the
Patchvalue at that key isNothing: delete that key from thePatchTarget. -
And the
Patchvalue at that key isJust x: update the value at that key in thePatchTargettox.
-
-
If the key is present in the
Patchand not present in thePatchTarget…-
And the
Patchvalue at that key isNothing: do nothing because we’re trying to delete a key that doesn’t exist in the target in the first place. -
And the
Patchvalue at that key isJust x: insert the key and the valuexinto thePatchTarget
-
-
If the key is not present in the
Patchbut present in thePatchTarget: do nothing.
There are, of course, more complicated ways of patching maps involving, for example, moving values from one key to another. You can find the code for that in Data.Patch.PatchMapWithMove. Note that the PatchTarget type associated with the PatchMapWithMove patch instance is still Map k v!