Extending Blocks

extends="parent" puts the code of another block in front of this one's, and runs the whole text as one script. That gives the definitions / action / tests split without a module system, in any language: the definitions block holds only functions, and each block that extends it adds the one thing it does with them — run the pipeline step, or test the functions.

The extended block is not run and not a dependency: nothing about it is executed for the extending block. Only its text is borrowed, so the extending block must still declare its own deps=, inputs= and outputs=.

Python

The definitions carry the interpreter=; blocks that extend them inherit it (and any env= the definitions list), so it is written once.

import re

def slug(text):
    return re.sub(r"[^a-z0-9]+", "-", text.lower()).strip("-")

The action adds one line to the definitions:

print(slug("Hello, Extending World!"))

The tests add theirs. They see every name the definitions define, private or not, because the two are one script:

import unittest

class SlugTests(unittest.TestCase):
    def test_lowercases_and_joins_words(self):
        self.assertEqual(slug("Hello World"), "hello-world")

    def test_drops_punctuation_at_the_edges(self):
        self.assertEqual(slug("  --Hi!--  "), "hi")

unittest.main(argv=["slug-test"], verbosity=2)

Shell

The same split works where there is no import at all. Extended blocks may be chained and are included once, in order, even when several paths reach them:

say() { printf '[%s] %s\n' "$1" "$2"; }

greet() { say greet "hello, $1"; }

greet "$(whoami)"

out="$(greet world)"
[ "$out" = "[greet] hello, world" ] || { echo "FAIL: $out"; exit 1; }
echo "ok"

Running it

meshfox examples/extends.canvas.md run py slug-test    # test the definitions
meshfox examples/extends.canvas.md run py slug         # run the action
meshfox examples/extends.canvas.md run sh greet-test

An error line number is a line of the joined text: the extended code comes first, so a line of the extending block is reported shifted by the length of what it extends. The definitions' own lines are exact.