aridity 17

DRY config and template system

aridity

The Arid Manifesto

• Paths as keys to avoid key concatenation
• It's never necessary to repeat a value
• Minimal syntax for surprise-free authoring
• Lazy context-sensitive evaluation
• Strongly (dynamically) typed values
• Central defaulting rather than at call sites
• Templating using same syntax as expressions
• Easy to correctly quote/escape values in templates
• Extensibility via user-defined functions
• Easy tasks are easy, hard tasks are possible
• Many applications can share one user config
• Principle of least astonishment driven design

Config file syntax

: Until aridity gets support for comments, you can use the colon directive to ignore data.
: Directives MUST be separated from data by whitespace, and are typically punctuation.

: Here's the equals directive:
foo = bar
: This does what you'd expect - assign the string value bar to foo.
: foo is actually a path of length 1, path components are whitespace-separated:
this is a path = this is a value
: Any existing assignment can be overridden:
foo = baz
this is a path = this is different

: Internal whitespace in values is preserved (leading and trailing whitespace is not):
two sentences = Some like 2 spaces.  After a full stop.

: You can use indentation to avoid typing a common path prefix multiple times:
app1 feature1
    data1 = value1
    data2 = value2
app2
    feature1 data = value3
    feature2
        data1 = value4
        data2 = value5
: Exactly the same effect without using indentation:
app1 feature1 data1 = value1
app1 feature1 data2 = value2
app2 feature1 data = value3
app2 feature2 data1 = value4
app2 feature2 data2 = value5

: The right hand side of an equals is actually an expression.
: In an expression, a dollar sign with brackets can be used to refer to another path:
has value
    bar = $(foo)
    value3 = $(app2 feature1 data)
: Round brackets and square brackets have exactly the same effect:
also has value bar = $[foo]
: Values can be concatenated:
two bars
    without spacing = $(foo)$(foo)
    with one space  = $(foo) $(foo)
    with 2 spaces   = $(foo)  $(foo)
: A few paths are predefined in every new context, such as:
home directory = $(~)

: To get a literal dollar there is a special form for quoting:
financial report = $'(We lost $100 on Friday.)
: Be careful with nested brackets, the first matching bracket ends the special form:
behaviour
    expected   = $'[Lunch cost $20 (worth it though).]
    unexpected = $'(Lunch cost $20 (worth it though).)

: Another special form can be used to preserve leading/trailing whitespace:
padded bars = $.( $(foo) $(foo) )
: Brackets can span multiple lines:
bar per line
    without final newline = $.($(foo)
$(foo))
    with final newline = $.($(foo)
$(foo)
)

: Evaluation is lazy, the expression is what is actually (and eagerly) assigned to the path:
no problem = $(this path will get a value later)
: If your use-case demands it, you can force eager evaluation:
bar even if foo changes later := $(foo)

: When evaluating a path the local context is examined first, then its parents if path not found:
host
    short path = nope
    guest short path = yep
    should be nope = $(short path)
    guest should be yep = $(short path)
does not work = $(short path)

: Use the dot directive to include config from another file:
. /path/to/other/config.arid
: Thus you can factor out any config that's common to multiple deployments, and override as needed.
: It's possible (but maybe not so useful) to include under a non-trivial path:
other stuff . /path/to/other/config.arid
: There is no default context for relative paths, you must set cwd up-front as inclusion is not lazy:
cwd = /path/to
. other/config.arid

: Text between dollar and open bracket (that isn't a special form) is a function name.
: A useful function predefined in every new context is the platform slash:
path = $/($(~) Desktop report.txt)
: Unlike most functions, / can also be used (less legibly) as a value:
path = $(~)$(/)Desktop$(/)report.txt
: All functions are first class objects that can be assigned and overridden in the usual ways:
slash := $(/)
/ = something else
path = $slash($(~) Desktop report.txt)

: Simple lists can be created using the plus equals convenience directive.
: Indentation means you don't have to repeat the directive for every list element:
years +=
    2018
    2019
years += 2020
: A predefined join function takes a list and a separator and does what you'd expect:
copyright = $join($(years) $.(, ))
: Observe that functions typically take values not identifiers, so you have to 'get' explicitly.
: Lists are just a special case of nested contexts, which are much more powerful:
person
    $.(The Guardians) year = 2018
    Greta year = 2019
summary = Person of the Year was $join($map($(person) $.($label() in $(year))) $.(, )).
: Here the predefined label function gives you access to the last path component of a list element.

Templates

• A template is simply an expression in a file, that may be quite large
• These are typically used to create config files for other languages e.g. YAML, HCL
  • Note that literal dollar signs must be quoted as above, everything else is safe
• A processtemplate script is provided for basic processing

processtemplate app.json.aridt <config.arid >app.json

• Conventionally the " path is set to the most useful escape function for the target format
  • Brackets can be elided in function composition e.g. $"$(key) is the same as $"($(key))