msfl_visitors

A visitor pattern based approach for converting MSFL to other forms

Usage

 # This is not actually working code yet! @todo revisit once Matt's refactor branch is merged.
 require 'msfl_visitor'

 filter = { make: "Toyota" }

 collector = String.new

 renderer = MSFLVisitors::Renderers::Chewy::TermFilter.new

 visitor = MSFLVisitors::Visitor.new collector, renderer

 MSFLVisitors::AST.new(filter).accept(visitor)

 => 'make == "Toyota"'

Architecture

msfl_visitors is designed to consume normalized Mattermark Semantic Filter Language (NMSFL). msfl_visitors implements a parser (parsers/msfl_parser) that converts NMSFL into an AST. msfl_visitors implements a visitor that traverses the ast and produces the well formed output. The behavior of the visitor is controlled through composition at construction. It accepts a collector and a renderer.

MSFLParser

The parser accepts a Hash containing NMSFL and produces an AST. The parser uses a simplified version of the visitor pattern to traverse the NMSFL and produce the AST.

Typically one does not interact with the parser directly, instead a consumer of this gem should interact with the AST.

AST

The abstract syntax tree that represents a certain query filter. In the version of the visitor pattern herein adopted, each node of the AST is responsible for managing its state and traversal of itself and children.

A consumer of this gem creates a new AST instance passing in a Hash of NMSFL. The AST will leverage the MSFL parser to construct itself. The AST object is a Node as it implements the #accept(visitor) method.

visitor

Unlike the classical visitor pattern double dispatch is not strictly achieved through type matching in the visitor. Instead the visitor is just a single service that is composed of a collector and a renderer. The double dispatch is codified inside of a renderer, which like the visitors in the classic pattern can produce multiple output DSLs.

collector

During traversal the output from each node needs to be stored or buffered somewhere. The collector serves this role. It can be as simple as a String or an Array, or it can be more elaborate. Ultimately it must respond to the shovel operator (<<)

renderer

The logic for rendering the AST nodes into the output DSL is codified in a renderer. The two principle renderers at this time are Chewy::TermFilter and Chewy:QueryStringFilter