YAMLSCRIPT

    Programming in YAML

 SYNOPSIS

    A YAMLScript program greet.ys to greet someone 6 times:

        #!/usr/bin/env yamlscript
    
        # Defined a function called 'main' (entry-point):
        main(name):       # main args come from ARGV
        - min =: 1        # Assign 1 to variable 'min'
        - max =:          # max = 6
            (-): [10, 4]  # 10 - 4 = 6
        - for:            # Loop over array, calling a function for each
          - (..): [$min, $max]     # Range operator (1-6)
          - greet: [ $_, $name ]   # Call the local 'greet' function
    
        # Define another function:
        greet(num,name):  # Takes 2 arguments
        - greeting =: Hello, $name!
        - say: $num) $greeting

    Run:

        $ yamlscript greet.ys YAMLScript
        1) Hello, YAMLScript!
        2) Hello, YAMLScript!
        3) Hello, YAMLScript!
        4) Hello, YAMLScript!
        5) Hello, YAMLScript!
        6) Hello, YAMLScript!

    Use the YAMLScript REPL:

        $ ys    # or 'ys --repl'
        ys> {range: [1, 3]}
        ---
        - 1
        - 2
        - 3
        ys> {say: Hello!}
        Hello!
        --- null
        ys> ---
        ..> say: Goodbye!
        ..> ...
        Goodbye!
        --- null
        ys> exit  # (ctrl-D)
        $

    See "Using the REPL" below.

 DESCRIPTION

    YAMLScript is a programming language that uses YAML as a base syntax.
    It feels like a YAML encoded Lisp, but with fewer parentheses. It takes
    inspiration from Clojure, Haskell and Perl.

    YAMLScript adds various scalar (valid YAML) syntax forms to make coding
    it clean and flexible.

    For instance you could use any of the following syntax forms to define
    and initialize the x variable to the number 42:

        - x =: 42           # Special DSL for the `def` function
        - def: [x, 42]      # Literal function call of `def`
        - (=): [x, 42]      # Use the operator alias of `def`
        - !def [x, 42]      # Use YAML tag instead of simgle pair mapping
        - != [x, 42]        # Operator alias tag
        - !expr [def,x,42]  # Tagged expr(ession)
        - (def x 42)        # Lisp/Clojure expression form (as YAML scalar)
        - (x = 42)          # Operator expr using Haskell style
        - ((=) x 42)        # Alternate Haskell style #1
        - (x `def` 42)      # Alternate Haskell style #2

    Take your pick. They are all valid YAML, they all compile to the same
    AST, they all evaluate to the same result.

    YAMLScript's runtime engine is whatever programming language you are
    using it from. If you are using the yamlscript or ys CLI binaries, it
    picks one for you. The current prototypes are written in Perl, Python
    and JavaScript.

    YAMLScript functions can be defined in the runtime language or in
    YAMLScript proper. YAMLScript has module support and they also can be
    written in either.

    This is a powerful concept because it lets you have clean,
    multilanguage front end code, that can do absolutely anything the
    runtime language is capable of.

    A good usage is writing tests. All of the YAMLScript tests are written
    in YAMLScript. The exact same test files are run, regardless of the
    runtime language they are testing, or the specific test framework in
    that language.

 INSTALLATION

    YAMLScript can be installed in several ways. Once installed you will
    have access to the yamlscript and ys CLI commands. You will also have
    library support to invoke YAMLScript directly from Perl, Python or
    JavaScript.

      * From the source repository

          $ git clone https://github.com/ingydotnet/yamlscript ~/.yamlscript
          $ source ~/.yamlscript/.rc    # Add this line to your shell rc file
          yamlscript --version

      * From CPAN

          cpanm YAMLScript

      * From PyPI

          pip install yamlscript

      * From NPM

          npm install @yaml/yamlscript

 YAMLSCRIPT LANGUAGE CAPABILITIES

      * Variable binding

          name =: world

      Variable names use lowercase letters a-z, digits 0-9 and must start
      with a letter. Name parts may be separated by a dash -.

      * Variable dereferencing

          the-value =: $name

      The $ sigil prefix is used to dereference a variable.

      * String interpolation

      Variable derefs are expanded in 'plain' (unquoted) YAML strings:

          - greeting =: Hello, $name!
          # Quoted strings are not interpolated
          - string =: 'Hello, $name!'
          # Unless tagged with '!'
          - greeting =: ! 'Hello, $name!'
          # Multiline strings need the tag
          - hi-bye =: ! |
            Hello, $name.
            Goodbye, $name.

      * Function calls

          say:
            join: [' ', Hello, world!]

      * Define functions

          square-and-add(x,y):
          - !expr [[$x, ^, 2], +, y]

      * Define multiple arity functions

          sum:
            (): 0
            (x): x
            (x, y): (x + y)
            (x, y, z*): (x + (y + (sum z*)))

      * Conditionals

          if:
          - (>): [x, 50         # condition
          - say: $x wins :)    # then
          - say: $x loses :(   # else

      or:

          if:
            (>): [x, 50         # condition
          then:
            say: $x wins :)    # then
          else:
            say: $x loses :(   # else

      * Try / Catch

          - try:
              (/): [42, 0]
            catch(e):
              say: Caught error '$e'

      * Iteration

          for(name):
          - [Alice, Bob, Curly]
          - say: Hello, $name!

      * Import Modules

          use:
          # Import 'foo.bar' namespace only:
          - Foo-Bar
          # Import all exported functions from module:
          - Some-Module: [+]
          # Import functions `this` and `that`
          - Another-Module: [this, that]
          # Import all except:
          - That-Module: [-one, -two]
          # Import a namespace using an alternate name
          - Your-String: my.str

      YAMLScript modules may be written in YAMLScript or in the runtime
      language.

      * Define YAMLScript Modules

      Modules are referred to (used) with the naming style Foo-Bar. They
      typically define a namespace foo.bar. Their file names depend on the
      language they are written in:

	* YAMLScript — foo/bar.ys

	* Perl — Foo/Bar.pm

	* Python — foo/bar.py or foo/bar/__init__.py

	* JavaScript — foo/bar.js or foo/bar/index.js

      To write a Foo-Bar module in YAMLScript:

          name: foo.bar
          # Define public functions
          f1(): ...     # Fully qualified as foo.bar/f1
          f2(x,y): ...
          f3(+):            # Multi-arity function
            (): ...         # 0 args
            (x): ...        # 1 arg
            (x,y): ...      # 2 args
            (x,y,z*): ...   # 3 or more args
          hide:             # Private/local functions. Not exportable.
            p1(): ...
            p2(): ...

 POSSIBLE USE CASES

    We will write example YAMLScript programs for each of these use cases,
    in time.

      * Programatic, full featured, software configurations.

      * Data driven unit testing with data and assertions in same file.

      * Simple template rendering using interpolated YAML literal scalars.

      * Write (acmeist) programs that can run in multiple programming
      languages.

      * Write modules that can be used natively from multiple programming
      languages.

      * Since programs are data, YAMLScript code bases can be refactored
      using data manipulation tools like jq.

 FUTURE PLANS

      * Implement most of the core libraries with YAMLScript

      * Programatically port many functions from clojure/core source code

      * Write an implementation spec for YAMLScript

      * Port YAMLScript to many languages

      * Write a Test Suite for YAMLScript (in YAMLScript)

      * Write a multi-language test framework in YAMLScript

      * Generate native programming language code from the YAMLScript AST

	* In multiple target programming languages

      * Write the YAML Reference Parser in YAMLScript

      * Write YAMLScript in YAMLScript

      * Spec the YAMLSchema (YAML Processor Config) language as a dialect
      of YAMLScript

 STATUS

    This is very ALPHA software.

    Some of the features documented here are not yet implemented:

      * YAMLScript REPL

      * Python Port (Very soon)

      * JavaScript Port (after Python)

      * Some functions abaove

    Everything in the test files definitely works. :)

 AUTHORS

      * Ingy döt Net ingy@ingy.net <mailto:ingy@ingy.net>

 COPYRIGHT AND LICENSE

    Copyright 2022 by Ingy döt Net

    This is free software, licensed under:

    The MIT (X11) License

