The Preserves Data Language

Preserves is a data model and serialization format. The Preserves model features the familiar boolean, integer, float, string, sequence, dictionaries, and set types. Less familiar to Nim are the symbol and record types. The symbol is a string that is elevated for use in type comparisons and the record is a labelled (usually by symbol) collection of fields.

The textual representation isn't necessary to study before using Preserves because the Preserves model is a subset of Nim types and a code-generator is available to convert Preserves schemas to Nim modules.

If you don't know why you need Preserves, see the Syndicate library.

Preserves schema

Here is an example schema for a chat protocol:

; Present is a record with symbol "Present" as record label
; and one string field referred to as "username".
Present = <Present @username string>.

; Says is a record with symbol "Says" as record label
; and two fields referred to as "who" and "what".
Says = <Says @who string @what string>.

Code Generation

The preserves-schema-nim utility would generate the following module for the preceding schema:

type
  Says* {.preservesRecord: "Says".} = object
    `who`*: string
    `what`*: string

  Present* {.preservesRecord: "Present".} = object
    `username`*: string

There are two types corresponding to the two records defined in the schema. The preservesRecord pragma allows for a lossless conversion between the Nim type system and Preserves records.

var
  present = Present(username: "Judy")
  pr = present.toPreserve()
assert $pr == """<Present "Judy">"""
assert present.fromPreserve(pr) == true

Library

To parse or produce Preserves one should write a schema and generate a Nim module using the preserves-schema-nim utility. This module will contain Nim types corresponding to schema definitions. The toPreserve andfromPreserve routines will convert Nim types to and from Preserves. The decodePreserves, parsePreserves, encode, and $ routines will convert Preserve objects to and from binary and textual encoding.

To debug the toPreserves and fromPreserves routines compile with -d:tracePreserves.

Utilities

• preserves-schema-nim
• preserves-encode
• preserves-decode
• preserves-from-json
• preserves-to-json
• preserves-from-xml
• preserves-to-xml

Installation

preserves_encode is a multi-call binary that implements preserves-encode, preserves-decode, preserves-from-json, and preserves-to-json, so the appropriate symlinks should be created during packaging.