YAML Embedding

Eucalypt can be embedded in YAML files via the following tags:

• eu
• eu::suppress
• eu::fn

The YAML embedding is not as capable as the native Eucalypt syntax but it is rich enough to be used for many YAML templating use cases, particularly when combined with the ability to specify several inputs on the command line.

Evaluating eucalypt expressions

As you would expect, YAML mappings correspond to Eucalypt blocks and bind names just as Eucalypt blocks do and YAML sequences correspond to Eucalypt lists.

YAML allow a wide variety of forms of expressing these (block styles and flow styles), to the extent that JSON is valid YAML.

Eucalypt expressions can be evaluated using the !eu tag and have access to all the names defined in the YAML unit and any others brought into scope by specifying inputs on the command line.

values:
  x: world
  y: hello

result: !eu "{values.y} {values.x}!"

...will render as:

values:
  x: world
  y: hello

result: Hello World!

Suppressing rendering

Items can be hidden using the eu::suppress tag. This is equivalent to :suppress metadata in the eucalypt syntax.

values: !eu::suppress
  x: world
  y: hello

result: !eu "{values.y} {values.x}!"

...will render as:

result: Hello World!

Defining functions

Functions can be defined using eu::fn and supplying an argument list:

values: !eu::suppress
  x: world
  y: hello
  greet: !eu::fn (h, w) "{h} {w}!"

result: !eu values.greet(values.y, values.x)

...will render as:

result: Hello World!

The escape hatch

Larger chunks of eucalypt syntax can be embedded using YAML's support for larger chunks of text, combined with !eu. Using this workaround you can access capabilities of eucalypt that are not yet available in the YAML embedding. (Although operators cannot be made available in YAML blocks because of the way that operator names are bound - see Operators and Identifiers.

block: !eu |
  {
    x: 99
    (l ^^^ r): "{l} <_> {r}"
    f(n): n ^^^ x
  }

result: block.f(99)