MapCast

A brave new tool to simplify the common task of transforming tree shaped data.

Vision

A power tool for managing JSON transformations. In REST interfaces, version changes could be programmatically provided by services in a CAST format. Elsewhere, this tool can be used to greatly facilitate mapping between ASTs.

CAUTION:

EXPERIMENTAL

Usage

Example Target JSON:

{
  "type": "Program",
  "range": [
    9,
    115
  ],
  "sourceType": "module",
  "loc": {
    "start": {
      "line": 4,
      "column": 0
    },
    "end": {
      "line": 7,
      "column": 3
    }
  },
  "body": [
    {
      "type": "ExpressionStatement",
      "range": [
        9,
        115
      ],
      "loc": {
        "start": {
          "line": 4,
          "column": 0
        },
        "end": {
          "line": 7,
          "column": 3
        }
      }
    }
  ]
}

Example Cast:

var transformedObject;
var exampleCast = [{
  pre: {
    loc: {
      'start': '$$',
      'end': '$$'
    },
    range: '$$'
  },
  post: {
    location: '$loc'
  }
}, 
{
  pre: {
    type: 'Program',
    body: '$$'
  },
  post: {
    'type': '$type',
    'children': ['$body']
  }
},
{
  pre: {
    sourceType: 'module',
    '$$': '$$'
  },
  post: {
    type: 'Module',
    children: ['$$']
  }
}]

  transformedObject = mapcast(exampleJSON, exampleCast)

{
  "type": "Module",
  "children": [
    {
      "type": "Program",
      "children": [
        {
          "type": "ExpressionStatement",
          "location": {
            "start": {
              "line": 4,
              "column": 0
            },
            "end": {
              "line": 7,
              "column": 3
            }
          }
        }
      ],
      "location": {
        "start": {
          "line": 4,
          "column": 0
        },
        "end": {
          "line": 7,
          "column": 3
        }
      }
    }
  ]
}

Explanation

Casts are objects with pairs of Pre and Post keys. The Pre functions as the precondition, if a subtree matches the precondition then it will transform it such that it matches the Post key.

For a Pre block to match a node, all keys of the Pre must exist in the node. Keys whose values are $$ will match to anything, while values that either do not start with '$' or are not strings will be used to match for value equivalence.

In a Post block, keys of $$ will extend the object with all the keys of source object. Values which begin with $ refer to paths on the source object, and simply copy the values. Finally, any value which does not begin with $ will be assumed to be the literal value provided.

Two gotchas:

$$ as a string in an array will be interpreted as the entire source object.
Any keys specified in the From block MUST BE SPECIFIED in the Post block, otherwise they will be discarded.

TODO