Improve AST manipulation options

[rmosolgo]

I think there are some good use cases for working with strings from Ruby:

• Programmatically building or modifying a query, as in graphql-client
• Merging queries together as in Merging GraphQL Queries Together #406
• Handling subscriptions efficiently (eg, unioning all queries into one, resolving it once, then distrubuting the requested parts of the response to the various subscribers)

However, this gem provides nothing for doing that. The best API would be one that doesn't reference AST node classes by name, instead, something decoupled from that, in case we have to change the nodes in the future.

I heard good things about estools a while back, there may be some inspiration there!

[rmosolgo]

GraphQL-JS supports this really nicely through their visitor API: http://graphql.org/graphql-js/language/#visit

For each kind of node, you may define an enter() and leave() function, and the return value of that function may affect the AST:

var editedAST = visit(ast, {
  enter(node, key, parent, path, ancestors) {
    // @return
    //   undefined: no action
    //   false: skip visiting this node
    //   visitor.BREAK: stop visiting altogether
    //   null: delete this node
    //   any value: replace this node with the returned value
  },
  leave(node, key, parent, path, ancestors) {
    // @return
    //   undefined: no action
    //   false: no action
    //   visitor.BREAK: stop visiting altogether
    //   null: delete this node
    //   any value: replace this node with the returned value
  }
});

I would love to do something like that, but to support immutable AST nodes, it could be a bit tricky!

[rmosolgo]

This will be available in 1.9 with #1740