Scala-inspired Embedded DSLs for Bash

Bash, by its nature, is a very "flat" language, that lacks the ability to cleanly define nested data structures (e.g. JSON, HTML, or any other structured content or configuration).

Enter scale-dsl: a bash 3.2+ micro-library that lets you create Structured Configuration And Language Extensions. By sourcing it, or copy-pasting it into your script, and then adding a few simple handler functions, it lets you write bash "outlines": nested data structures and commands, whose semantics are defined by bash functions.

For example, using the handler functions in demos/scale-json, this bash function:

contact-as-json() {
  ~ json map; {{
    - first_name str "$1"
    - last_name  str "$2"
    - email      str "$3"
    + tags list; {{
        local tag
        for tag in "${@:4}"; do
            - str "$tag"
        done
    }}
  }}
}

can be run to generate valid JSON for arbitrary arguments:

    $ contact-as-json Bob Dobbs [email protected] fake demo
    $ echo "$REPLY"
    {"first_name":"Bob", "last_name":"Dobbs", "email":"[email protected]", "tags":["fake", "demo"]}

(The above code is pretty fast, too: generating "Bob Dobbs"'s JSON takes about 1/800th of a second on a modest Linux server: less time than it would take to even start an external command!)

Of couse, you're not limited to producing JSON, or even text. Your outline items (and block-accepting functions) can set variables, run programs, or do whatever other kind of computation is required. It's entirely up to you (and your outline handlers).

Syntax

SCALE works by adding just three new syntactic structures to bash:

• ~ shell-command(s)... ; {{ block... }}

  Turns block into a function that one of the shell-commands can call (or not), optionally setting a handler for nested outline items.

• - outline-item-args...

  Sends outline-item-args as arguments to the current outline handler, to be interpreted as data or as a directive/subcommand.

• + outline-item-args... ; {{ block... }}

  Sends outline-item-args as arguments to the current outline handler, which can call block (or not), optionally changing the handler for nested outline items. (The + is literally just shorthand for ~ -: i.e., wrapping an outline item as a block-taking command.)

So in the example code above, json map is invoking the shell function json with the single argument map, and then that function invokes the outermost block with an outline handler that processes outline items as JSON object entries (a key, a JSON type, and an optional value). The map and list data types process nested blocks in this fashion, while other data types' values can be specified without a nested block. (Also notice that - and + are simple bash commands and as such can be enclosed in loops or conditionals, use variable interpolation, etc.)

Basic Use

The basic idea of SCALE outlines is that a ~-prefixed command series controls when (or whether) to run the block that follows it, optionally setting an "outline handler" that will be called whenever a + or - outline item is executed within that block. For example, here's the simplest possible outline handler you can use with SCALE:

    $ source scale-dsl  # load the library
    $ ~ ::block echo; {{ - Hello; - World; }}
    Hello
    World

In this example, the command ::block echo tells SCALE to run the block and pass any - or + outline items to echo (our outline handler) as arguments. So Hello and World both get echoed, as each - is expanded at runtime to echo.

Now let's try a slightly more complex example, with nesting, and passing prefix arguments to outline handler functions:

    # Set the initial indent
    $ indented() { ::block indent-with ""; }

    $ indent-with() {
    >     # Output the indent and the input
    >     local indent=$1; shift; echo "$indent$*"
    >     # Got a block?  Run it with a deeper indent
    >     ::block indent-with "    $indent"
    > }

    $ ~ indented; {{
    >   - This is a test
    >   + of indenting ; {{
    >     - nested
    >     + deeper ; {{
    >         for i in 1 2 3; do
    >             - yeah "($i)"
    >         done
    >     }}
    >     - back out
    >   }}
    >   - and again
    > }}
    This is a test
    of indenting
        nested
        deeper
            yeah (1)
            yeah (2)
            yeah (3)
        back out
    and again

Each basic command in the block that begins with - or + is forwarded to the outline handler (indent-with "") as command line arguments. So the line - This is a test becomes indent-with "" This is a test, and + of indenting becomes indent-with "" of indenting. The indent-with function then runs any nested block with a deeper indent.

(As you can see, ::block accepts any number of arguments, not just a command or function name. This makes it easier to create reusable handler functions, as the extra arguments are included in each expansion of + and -.)

Scripting and Composition

Notice that SCALE blocks are still plain bash code, and can therefore use control structures, expressions, parameter expansion, and all other normal commands or functions, in addition to the ones prefixed by +, -, or ~. Outline handlers are inherited even across function calls, so you can put parts of a data structure into functions and then reuse them from different enclosing structures and even different handlers, e.g.:

    $ say-hello() { - "Hello, $1!"; }

    $ ~ ::block echo; {{ say-hello world; }}
    Hello, world!

    $ ~ indented; {{
    >   + "My response:"; {{ say-hello yourself; }};
    > }}
    My response:
        Hello, yourself!

Notice how the - in the say-hello function produces different results in each use, expanding to echo or indent-with " ", according to where it's called from. (Notice, too, that we invoked it as a normal bash function, not as a - line unto itself. If we had put a - in front of it, we would have ended up calling echo say-hello world and indent-with " " say-hello yourself, which would have produced rather different results!)

Although you can separate what's inside a {{...}} block into different functions, do note that you can't separate a block from its opening + or ~: they must be immediately adjacent and within the same enclosing bash block or control structure. (Otherwise, a bash syntax error will occur.)

(Note, too that a + or ~ and its block are invisibly wrapped togethr as a bash compound statement; so e.g. any redirection applied after the closing }} will apply to the execution of the commands or outline item preceding the {{ part.)

Variables and Parameters

Blocks within a function can use that function's received arguments normally ($1, $2, etc.), regardless of how deep the block nesting. Unlike normal bash blocks, however, SCALE blocks can actually have their own local variables, shadowing the ones defined by the function.

Within any given block, local variables from any enclosing blocks or containing/calling functions are also accessible, alternating in precedence with the locals of the outline handler(s) or top-level command that invoked each block. So, within some nested block, the order of variable lookup or assignment would be:

• $1, $2... etc. from the enclosing function or script (no matter how deeply nested the blocks)
• locals defined within that block
• locals defined by the handler that called ::block to run the block
• locals defined by the enclosing block
• locals defined by the handler that called ::block to run the enclosing block
• ...
• locals defined by the command that called ::block on the outermost block
• locals defined outside the outermost block (e.g. the enclosing function, if any)
• locals defined by the caller(s)
• globals

This order applies to the handlers, too, in that they can see local variables defined below themselves in this list. (But their positional parameters are always their own: they see their own arguments in $1, $2, etc., not those of the blocks' enclosing function.)

Controlling Block Execution and Interpretation

If you're designing a DSL or API, note that calling ::block from a ~ command or + item is strictly optional; if you don't call it, the code within {{...}} simply won't be executed. A given block can only be executed once, however: calling ::block more than once for the same block will have no effect (unless the calls are done inside a subshell). Calling ::block with no arguments runs the block without changing the current outline handler.

If no outline handler is defined (i.e. if you use + or - without an enclosing block), a handler called ::no-dsl will be invoked (if it exists). If your program does not have a ::no-dsl function and a ::no-dsl command does not exist on your PATH, an error will occur.

Compatibility Notes

SCALE is implemented mainly using bash aliases (for + , -, ~, {{ and }}), and the public ::block function. Internally, however, SCALE reserves three additional function names for its own use (aside from ::no-dsl), and has several private variables. To avoid unwanted and unpredictable behavior, you should not use or define functions named ::^, ::, or ^::, nor should you set, unset, or declare variables named __blk__, __bsp__, or __dsl__.

For optimum performance, SCALE is carefully written to avoid forking. However, if a block-taking function or outline handler uses blocks itself (or calls other code that does) before the enclosing block is executed, a $(declare -f ::) substitution is required to save the source code of the not-yet-executed block. You can avoid this overhead by ensuring that any other block-using code is run after your code calls ::block.

SCALE syntax is mostly compatible with shellcheck, except that you need to disable SC2215 (Commands beginning with -) and SC1054 (doubled braces). Adding # shellcheck disable=1054,2215 on the line before a block begins (or the enclosing function, if any) will disable them locally, or you can disable them at the project level if you prefer.

Finally, note that if you need to run actual programs or functions named + , -, ~, {{ or }}, you can simply prefix them with a \ or enclose them in quotes. This will stop the shell from expanding them as aliases. If the code can't easily be changed, you can simply source it before scale-dsl, or else disable the expand_aliases shell option while sourcing it, and re-enable it afterward. Bash expands aliases while code is being compiled, not when it's run, so you can have DSL-using and non-DSL-using functions in the same program, with different understandings of what -, +, etc. mean.

(Also, if you're using bash 4.4's new local - construct, please note that it has a bug that disables alias expansion in scripts. If you call such a function before all of your source code has been read, you'll need to re-enable alias expansion, outside the function, after the call is made.)

License

To the extent possible under law, PJ Eby has waived all copyright and related or neighboring rights to bashup/scale-dsl. This work is published from: United States.