patch: Data structures for describing changes to other data structures.

Data structures for describing changes to other data structures.

In this library, a patch is something which can be applied, analogous to a function, and which distinguishes returning the argument it was provided from returning something else.

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Versions [RSS]	0.0.0.0, 0.0.0.1, 0.0.1.0, 0.0.2.0, 0.0.3.0, 0.0.3.1, 0.0.3.2, 0.0.4.0, 0.0.5.0, 0.0.5.1, 0.0.5.2, 0.0.6.0, 0.0.7.0 (info)
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Dependencies	base (>=4.9 && <4.15), constraints-extras (>=0.3 && <0.4), containers (>=0.6 && <0.7), dependent-map (>=0.3 && <0.5), dependent-sum (>=0.6 && <0.8), lens (>=4.7 && <5), monoidal-containers (==0.4.0.0 \|\| >=0.6 && <0.7), semialign (>=1 && <1.2), semigroupoids (>=4.0 && <6), these (>=0.4 && <0.9 \|\| >=1 && <1.2), transformers (>=0.5.6.0 && <0.6), witherable (>=0.3 && <0.4) [details]
License	BSD-3-Clause
Author	Ryan Trinkle
Maintainer	maintainer@obsidian.systems
Revised	Revision 2 made by abrar at 2021-04-16T00:59:42Z
Category	FRP
Home page	https://obsidian.systems
Bug tracker	https://github.com/reflex-frp/patch/issues
Source repo	head: git clone https://github.com/reflex-frp/patch
Uploaded	by abrar at 2020-11-06T13:21:39Z
Distributions	NixOS:0.0.6.0
Downloads	3776 total (124 in the last 30 days)
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Readme for patch-0.0.3.2

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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 Patch and the PatchTarget...
- And the Patch value at that key is Nothing: delete that key from the PatchTarget.
- And the Patch value at that key is Just x: update the value at that key in the PatchTarget to x.
If the key is present in the Patch and not present in the PatchTarget...
- And the Patch value at that key is Nothing: do nothing because we're trying to delete a key that doesn't exist in the target in the first place.
- And the Patch value at that key is Just x: insert the key and the value x into the PatchTarget
If the key is not present in the Patch but present in the PatchTarget: 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!