RFC 6902 "JSON Patch" implemented in `ytt`

config.yml

---
kind: Package
spec:
  template:
    spec:
      fetch:
        - imgpkgBundle:
            image: base-image

jsonpatch.star

load("@ytt:struct", "struct")
load("@ytt:yaml", "yaml")

# `raw_yaml()` makes an editable copy of `node`
#   warning: all nodes are scrubbed of attached annotations and code.
def raw_yaml(node):
  return yaml.decode(yaml.encode(node))
end

def key_or_index(target, part):
  if type(target) == type([]):
    part = len(target) if part == "-" else int(part)
  end
  return part
end

# `at()` returns:
# 1. the penultimate descendant located along `path` from `node`
# 2. the key/index to the ultimate descenant
def at(node, path):
  parts = path.split("/")[1:]
  target = node
  for part in parts[:-1]:
    target = target[key_or_index(target, part)]
  end
  return (target, key_or_index(target, parts[-1]))
end

def edit(left, path, right, op):
  copy = raw_yaml(left)
  (target, key) = at(copy, path)
  op(target, key, right)
  return copy
end

def add(target, key, right):
  if type(target) == type([]):
    target.insert(key, right)
  end
  target[key] = right
end

def remove(target, key, _):
  target.pop(key)
end

def replace(target, key, right): 
  target[key] = right
end

def move(left, src, dest):
  (target, key) = at(raw_yaml(left), src)
  copy = edit(left, src, None, remove)
  copy = edit(copy, dest, target[key], add)
  return copy
end

def jsonpath_add(path):
  return lambda left, right: edit(left, path, right, add)
end

def jsonpath_remove(path):
  return lambda left, right: edit(left, path, right, remove)
end

def jsonpath_replace(path):
  return lambda left, right: edit(left, path, right, replace)
end

def jsonpath_move(src, dest):
  return lambda left, right: move(left, src, dest)
end

jsonpatch = struct.make(add=jsonpath_add, move=jsonpath_move, remove=jsonpath_remove, replace=jsonpath_replace)

overlay.yml

#@ load("@ytt:overlay", "overlay")
#@ load("jsonpatch.star", "jsonpatch")

#@overlay/match by=overlay.subset({"kind": "Package"})
#@overlay/replace via=jsonpatch.replace("/spec/template/spec/fetch/0/imgpkgBundle/image")
---
dev_image

rfc6902_examples.yml

#@ load("@ytt:overlay", "overlay")
#@ load("jsonpatch.star", "jsonpatch")

---
id: A.1
name: Adding an Object Member
foo: bar

#@overlay/match by=overlay.subset({"id": "A.1"})
#@overlay/replace via=jsonpatch.add("/baz")
---
qux

#@overlay/match by=overlay.subset({"id": "A.1"})
#@overlay/assert
---
baz: qux
foo: bar
...


---
id: A.2
name: Adding an Array Element
foo:
- bar
- baz

#@overlay/match by=overlay.subset({"id": "A.2"})
#@overlay/replace via=jsonpatch.add("/foo/1")
---
qux

#@overlay/match by=overlay.subset({"id": "A.2"})
#@overlay/assert
---
foo:
#@overlay/assert
- bar
#@overlay/assert
- qux
#@overlay/assert
- baz
...

---
id: A.3
name: Removing an Object Member
baz: qux
foo: bar

#@overlay/match by=overlay.subset({"id": "A.3"})
#@overlay/replace via=jsonpatch.remove("/baz")
---

#@overlay/match by=overlay.subset({"id": "A.3"})
#@overlay/assert
---
foo: bar
...



---
id: A.4
name: Removing an Array Element
foo: ["bar", "quz", "baz"]

#@overlay/match by=overlay.subset({"id": "A.4"})
#@overlay/replace via=jsonpatch.remove("/foo/1")
---

#@overlay/match by=overlay.subset({"id": "A.4"})
#@overlay/assert
---
foo:
#@overlay/assert
- bar
#@overlay/assert
- baz
...


---
id: A.5
name: Replacing a Value
baz: qux
foo: bar

#@overlay/match by=overlay.subset({"id": "A.5"})
#@overlay/replace via=jsonpatch.replace("/baz")
---
boo

#@overlay/match by=overlay.subset({"id": "A.5"})
#@overlay/assert
---
baz: boo
foo: bar
...



---
id: A.6
name: Moving a Value
foo:
  bar: baz
  waldo: fred
qux:
  corge: grault

#@overlay/match by=overlay.subset({"id": "A.6"})
#@overlay/replace via=jsonpatch.move("/foo/waldo","/qux/thud")
---

#@overlay/match by=overlay.subset({"id": "A.6"})
---
foo:
  #@overlay/assert
  bar: baz
qux:
  #@overlay/assert
  corge: grault
  #@overlay/assert
  thud: fred
...

---
id: A.7
name: Moving an Array Element
foo:
- all
- grass
- cows
- eat

#@overlay/match by=overlay.subset({"id": "A.7"})
#@overlay/replace via=jsonpatch.move("/foo/1", "/foo/3")
---

#@overlay/match by=overlay.subset({"id": "A.7"})
---
#@overlay/assert
foo:
#@overlay/assert
- all
#@overlay/assert
- cows
#@overlay/assert
- eat
#@overlay/assert
- grass
...

---
id: A.10
name: Adding a Nested Member Object
foo: bar

#@overlay/match by=overlay.subset({"id": "A.10"})
#@overlay/replace via=jsonpatch.add("/child")
---
grandchild: {}

#@overlay/match by=overlay.subset({"id": "A.10"})
---
#@overlay/assert
foo: bar
#@overlay/assert
child:
  #@overlay/assert
  grandchild: {}
...

---
id: A.16
name: Adding an Array Value
foo:
- bar

#@overlay/match by=overlay.subset({"id": "A.16"})
#@overlay/replace via=jsonpatch.add("/foo/-")
---
- abc
- def

#@overlay/match by=overlay.subset({"id": "A.16"})
---
foo:
#@overlay/assert
- bar
#@overlay/assert
- 
  #@overlay/assert
  - abc
  #@overlay/assert
  - def
...