JEP-14 Compact syntax for multi-select-hash

[springcomp]

Compact syntax for multi-select-hash

JEP	14
Author	Dan Vanderkam, Maxime Labelle
Status	draft
Created	26-Feb-2022

Abstract

This JEP proposes a grammar modification to support
ES6-style
compact multi-select-hash constructs.

Motivation

Consider the following JSON document:

{
  "values": [
    {"id": 1, "first": "Bill", "last": "Gates"},
    {"id": 2, "first": "Larry", "last": "Page"}
  ]
}

Let’s say we wanted to return a hash containing ids and first names.
This is currently possible using the following syntax:

values[*].{id:id, first:first}

Since {x:x, y:y} is such a common pattern, ES6 introduced compact object literal notation to simplify this. In ES6, {x,y} is equivalent to {x:x, y:y}.

This JEP supports this syntax to make JMESPath usage slightly less verbose.

Specification

What is currently the multi-select-hash production will be renamed
to multi-select-hash-keyval:

multi-select-hash_keval = "{" ( keyval-expr *( "," keyval-expr ) ) "}"

The multi-select-hash production will be modified slightly to accomodate
the syntax changes:

multi-select-hash	= multi-select-hash-key
multi-select-hash	=/ multi-select-hash-keyval

The new multi-select-hash-key production is defined like so:

multi-select-hash-key = "{" ( identifier *( "," identifier ) ) "}" 

Here is the full update to the grammar:

multi-select-hash        = multi-select-hash-key
multi-select-hash        =/ multi-select-hash-keyval

multi-select-hash-key    = "{" ( identifier *( "," identifier ) ) "}" 
multi-select-hash-keyval = "{" ( keyval-expr *( "," keyval-expr ) ) "}"

Motivating Example

With these changes defined, the expression in the “Motivation” section can be
be written as:

values[*].{id, first}

Which evaluates to [{"id": 1, "first": "Bill"}, {"id": 2, "first": "Larry"}].

Rationale

This JEP is inspired by an idea
from Dan Vanderkam.