Foundation · Libraries · Uncategorized

A YAML Library for Delphi

YAML has become a quite popular human-readable data serialization language. Unfortunately, there aren’t many Delphi libraries that provide tools for it. I hope to fill this void with Neslib.Yaml, a personal project build on top of LibYaml that provides support for parsing and generating YAML documents and streams.

LibYaml Bindings

Neslib.Yaml uses LibYaml to perform the actual parsing and emitting. The Neslib.LibYaml unit contains the header translations in case you want to use LibYaml directly. However, the Neslib.Yaml unit provides a much easier interface for working with YAML. You can use it to load or parse a YAML document into a DOM. You can also build your own DOM and convert it to YAML.

On the Windows platform, LibYaml is linked into the executable through a set of object (.obj) files. So there is no need to deploy any DLLs. The same goes for iOS and Android, where LibYaml is linked as a static library. For 32-bit macOS, you need to deploy a dylib though. 64-bit macOS and Android support will follow later when Delphi has compilers for these platforms. The current version does not support Linux or the iOS simulator.

YAML in a Nutshell

Here is a very brief introduction for YAML. For more detailed information take a look at the official YAML site or one of the many on-line resources such as this one. Feel free to skip this section if you are already familiar with YAML.

YAML (short for “YAML Ain’t Markup Language”) is a data serialization language. Unlike many other similar text-based languages (like JSON and XML) a primary goal of YAML is to be human-readable and also easy to write by humans. That’s why its is commonly used for configuration files. However, it can be used for all kinds of data, such as this example invoice from the YAML specification:

invoice: 34843
date   : 2001-01-23
bill-to: &id001
    given  : Chris
    family : Dumars
    address:
        lines: |
            458 Walkman Dr.
            Suite #292
        city    : Royal Oak
        state   : MI
        postal  : 48046
ship-to: *id001
product:
    - sku         : BL394D
      quantity    : 4
      description : Basketball
      price       : 450.00
    - sku         : BL4438H
      quantity    : 1
      description : Super Hoop
      price       : 2392.00
tax  : 251.42
total: 4443.52
comments: >
    Late afternoon is best.
    Backup contact is Nancy
    Billsmer @ 338-4338.

A YAML document is a tree of values, called nodes (TYamlNode in this library). There are 4 kinds of nodes:

Mappings

Mappings are similar to Delphi dictionaries. A mapping is a collection of key/value pairs. The root note of the sample document above is a mapping: it maps the key invoice to the value 34843 and contains 7 other key/value pairs (from date to comments). Both keys and values can be any YAML type, although you probably want to stick to strings for keys.

Mappings can be written in block notation (as in the example) or flow notation (using curly braces {}).

When using block notation, YAML uses indentation for scoping. Only spaces are allowed for indentation (not tabs). The number of spaces doesn’t matter as long as all values at the same level use the same amount of spaces. In the example, the value of the bill-to key is another mapping (ignore the &id001 for now). This mapping is indented to indicate that it is the value of the bill-to key.

Sequences

Sequences are like Delphi arrays or lists. Small sequences can be written using flow notation (using square brackets []). Larger or complex sequences are usually written in block notation as in the example: the value of the product key is a sequence of two products (a basketball and super hoop). Each item in the sequence starts with a dash and a space.

In this example, each product in the sequence is a mapping of 4 key/value pairs.

Aliases

All nodes have at least two properties: a Tag and an Anchor. Tags are used to describe the semantic type of a node. Tags are not that common, so I will skip them in this introduction. Neslib.Yaml has full support for tags though.

An anchor can be used to mark a node in the document. You can then later refer back to this node using an alias.

Anchors are prefixed with an ampersand (&). In the example, the value of the bill-to key has an anchor called id001 (the ampersand is not part of the name). Later in the document, the ship-to key refers back to this anchor using an alias (an asterisk followed by the anchor name, eg. *id001). This is a way of saying that the shipping address is the same as the billing address. Note that an alias does not copy the referenced value; it really just refers to another node.

Anchors must appear in the document before they can be referenced. Their names don’t have to be unique within the document; if an new anchor is declared with the same name, it replaces the old anchor.

Scalars

Scalars are the simplest types. Everything that is not a mapping, sequence or alias is a scalar. In practice, scalars are just strings. All the keys in the example above are string scalars, but a lot of the values are as well (such as 34843, 2001-01-23 and Chris).

The YAML 1.1 specification (which is what LibYaml uses) treats all these scalars as strings, even if they are numbers or dates as in this example. You can use tags to explicitly state that a specific scalar is of a specific type.

The TYamlNode record in this library provides methods like ToInteger and ToDouble to (try to) convert node values to Delphi types, regardless of any tags that may be attached to a node.

Scalars can be written in different “styles”:

  • The plain style is the most common style. It doesn’t use any special symbols. Most scalars in the example are in plain style.
  • The double-quoted style is useful if you need escape sequences in the text.
  • The single-quoted style can be used if backslashes in text should not be un-escaped (eg. when using Windows file paths).
  • The literal style can be used for a block of text spanning multiple lines. It starts with a pipe symbol (|). In the example above, the bill-to.address.lines value is a literal. Any new-lines in a literal are preserved.
  • Finally, the folded style is similar to the literal style, but line breaks are folded (replaced with spaces). It is used with the comments key in the example.

There is much more to YAML, but this should cover many use cases.

Loading or Parsing YAML

The main entry point to the Neslib.Yaml library is the IYamlDocument or IYamlStream interface.

A YAML file can contain multiple documents. If that is the case, you should use an IYamlStream to load it. A stream is just a collection of documents (of type IYamlDocument).

Most of the time though, a YAML file contains just a single document and it is easier to start with a IYamlDocument. Loading a document is easy:

var
  Doc: IYamlDocument;
begin
  Doc := TYamlDocument.Load('invoice.yaml');
end;

You can load from a file or stream, or you can parse YAML text using the TYamlDocument.Parse method.

You can now use the IYamlDocument.Root property to inspect the document. This property is of type TYamlNode, which is the building block for all documents.

TYamlNode is implemented as a record to keep it light-weight. All nodes are “owned” by a document. This makes memory management fully automatic: once a document goes out of scope, all its nodes will be freed automatically. This does mean though that you should not “hang on” to nodes after a document has gone out of scope. Doing so will result in undefined behavior or access violations.

For example, to access the price of the first product in the example above, you can use the following code:

Price := Doc.Root.Values['product'].Nodes[0].Values['price'].ToDouble;

You use the Values property to access values by key in mapping. Likewise the Nodes property is used to access values by index in a sequence, and one of the ToXXX methods can be used to convert a scalar value to a Delphi datatype.

None of these properties and methods raise exceptions. For example, when you use the Values property on a node that isn’t a mapping, or the key that you specified doesn’t exist, then it will return a special Null node instead of raising an exception. Accessing this Null node also doesn’t raise exceptions. It just doesn’t do anything. This allows you to write expressions like the one above without having to check every intermediate step.

Of course, in many situations you still want to check the data before accessing it. To check the type of a node, you can use the NodeType property or one of the IsXXX properties (IsMapping, IsScalar etc.). To check if a key exists, you can use Contains or TryGetValue. There are other properties and methods you can use for validation as well.

Constructing and Emitting YAML

You can also create a YAML document from scratch and save it to a file or convert it to YAML. To create a YAML document, use one of the TYamlDocument.CreateXXX methods, depending on the type of root node you need. If you want to reconstruct the example document, you would start out with a mapping and call:

Doc := TYamlDocument.CreateMapping;

You can then start to add key/value pairs:

Doc.Root.AddOrSetValue('invoice', 34843);
Doc.Root.AddOrSetValue('date', '2001-01-23');

The AddOrSetValue method is used to add key/value pairs to a mapping.

Unlike the properties and methods discussed earlier, the “building” methods do raise exceptions when used incorrectly. In this case, if the node is not a mapping, then an EInvalidOperation exception will be raised when using the AddOrSetValue method.

To add a non-scalar value, use one of the other AddOrSetXXX methods:

var
  Products: TYamlNode;
begin
  Products := Doc.Root.AddOrSetSequence('product');
end;

This adds a sequence to the mapping with the key product. You can then add values to the sequence using one of the AddXXX methods. Again, an EInvalidOperation exception will be raised if the node is not a sequence. In the example, we need to add another mapping to this sequence:

var
  Product: TYamlNode;
begin
  Product := Products.AddMapping;
  Product.AddOrSetValue('sku', 'BL394D');
  Product.AddOrSetValue('quantity', 4);
  // etc...
end;

Once you have constructed your document, you can save it to a file or stream using the Save method, or convert it to YAML using the ToYaml method:

var
  Yaml: String;
begin
  Yaml := Doc.ToYaml;
end;

You can pass an optional TYamlOutputSettings record to customize the YAML formatting.

More Information

You can find the Neslib.Yaml repository on GitHub.

This repository depends on the Neslib repository, which is provided as a sub-module. So be sure to clone the repository recursively:

git clone --recursive https://github.com/neslib/Neslib.Yaml

There is more to Neslib.Yaml than described above. For more details you can look at the well-document Neslib.Yaml.pas source file. Additional usage samples can be found in the unit tests, especially in the Tests.Neslib.Yaml.Sample.pas file.

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