Querying CouchDB with human-readable syntax

tango provides a C-like syntax for logical and relational expressions, parsing them directly into Mango selectors.

In my previous post, I introduced couchilla, a command-line tool for bundling CouchDB design documents. While MapReduce views are powerful, the /{db}/_find API typically provides faster querying in most cases by using Mango selector expressions.

For instance, in Mango, you'd express a query for "movies directed by Roy Andersson after 2007" with this JSON structure:

{
  "$and": [
    {
      "director": {
        "$eq": "Roy Andersson"
      }
    },
    {
      "year": {
        "$gt": 2007
      }
    }
  ]
}

... which is, as you can see, a hand-written AST!

With tango, you could write this instead:

director == "Roy Andersson" && year > 2007

This syntax includes standard C operators and supports parentheses to specify explicit precedence. At present, tango does not support unary operators or the $in operator for matching elements within arrays.

📄 See the lexer implementation in scan.js.

To parse Tango is to turn it into Mango

The shunting yard algorithm is a linear-time algorithm for parsing expressions using operator precedence parsing. It employs a stack to manage operators and a queue to construct an abstract syntax tree, which, in this context, becomes a Mango expression.

📄 See the parser implementation in parse.js.