SWI-Prolog YAML library
Jan Wielemaker
VU University Amsterdam
CWI, Amsterdam
The Netherlands
E-mail: J.Wielemaker@vu.nl
Abstract
This package reads and writes YAML documents from and to SWI-Prolog streams, files and strings. It is based on libyaml. This C library is being used by several languages. Using this C library provides good performance, and interoperability with YALM infrastructure used by other systems.

# Table of Contents

## 1 library(yaml): Process YAML data

This module parses YAML serialized data into a Prolog term with structure that is compatible with the JSON library. This library is a wrapper around the C library `libyaml`. This library forms the basis of the YAML support in several languages and thus guarantees compatibility of our YAML support with other languages.

[det]yaml_read(+Input, -DOM)
  Parse Input to a YAML DOM. The DOM representation uses the following mapping:

  - A YAML sequence is mapped to a Prolog List.
  - A YAML mapping is mapped to a Prolog dict.
  - Untagged scalars follow the implicit tag rules defined by YAML, providing numbers (int, float and special floats), `null` and the booleans `true` and `false`. Other untagged values are returned as a Prolog string. Tagged values are returned as `tag(Tag, String)` which is processed by yalm_tagged/3. This internal predicate calls the user hook yaml:tagged/3 with the same arguments and, if the hook fails, provides the following defaults:

    - `!!binary` converts the Base64 to a string of bytes.
    - `!!str` explicitly keeps a string
    - `!!null` translates "null" to `null`
    - `!!bool` translates to `true` and `false`
    - `!!int` translates to an integer
    - `!!float` translates to a float
    - Anything else is returned as `tag(Tag, String)`

  Input | is one of (1) a stream, (2) a term `string(Data)` or (3) a file name.

  bug
    YAML defines that floats do not require a digit after the decimal dot. We use the Prolog parser which does require the decimal dot to be followed by at least one digit. Because the YAML spec intends to match JSON which does require a digit, we ignore this incompatibility, expecting it will be addressed in the next YAML version.
[det]yaml_write(+Out:stream, +DOM)
[det]yaml_write(+Out:stream, +DOM, +Options)
  Emit a YAML DOM object as a serialized YAML document to the stream Out. Options processed are:
  canonical(+Boolean)
    Use canonical representation. Default is `false`.
  unicode(+Boolean)
    Use unicode Default is `true`.
  implicit(+Boolean)
    Use implicit or explicit representation. Currently only affects the opening and closing the document. Default is `true`. Use `false` for embedded documents.
  factorize(+Boolean)
    If `true`, minimize the term by factoring out common structures and use `&anchor` and `*anchor`. Factorization is always used if DOM is a cyclic term.
[semidet,multifile]tagged(+Tag, ?String, ?Value)
  Hook that allows convering `!!tag` values to be decoded or encoded.

# Index

?
tagged/3
yaml_read/2
yaml_write/2
yaml_write/3