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tbsp - tree-based source-processing language
tbsp is an awk-like language that operates on tree-sitter
syntax trees. to motivate the need for such a program, we
could begin by writing a markdown-to-html converter using
tbsp and tree-sitter-md [0]. we need some markdown to begin
with:
# 1 heading
content of first paragraph
## 1.1 heading
content of nested paragraph
for future reference, this markdown is parsed like so by
tree-sitter-md (visualization generated by tree-viz [1]):
document
| section
| | atx_heading
| | | atx_h1_marker "#"
| | | heading_content inline "1 heading"
| | paragraph
| | | inline "content of first paragraph"
| | section
| | | atx_heading
| | | | atx_h2_marker "##"
| | | | heading_content inline "1.1 heading"
| | | paragraph
| | | | inline "content of nested paragraph"
onto the converter itself. every tbsp program is written as
a collection of stanzas. typically, we start with a stanza
like so:
BEGIN {
int depth = 0;
print("<html>\n");
print("<body>\n");
}
the stanza begins with a "pattern", in this case, "BEGIN",
and is followed a block of code. this block specifically, is
executed right at the beginning, before traversing the parse
tree. in this stanza, we set a "depth" variable to keep
track of nesting of markdown headers, and begin our html
document by printing the "<html>" and "<body>" tags.
we can follow this stanza with an "END" stanza, that is
executed after the traversal:
END {
print("</body>\n");
print("</html>\n");
}
in this stanza, we close off the tags we opened at the start
of the document. we can move onto the interesting bits of
the conversion now:
enter section {
depth += 1;
}
leave section {
depth -= 1;
}
the above stanzas begin with "enter" and "leave" clauses,
followed by the name of a tree-sitter node kind: "section".
the "section" identifier is visible in the
tree-visualization above, it encompasses a markdown-section,
and is created for every markdown header. to understand how
tbsp executes above stanzas:
document ... depth = 0
| section <-------- enter section (1) ... depth = 1
| | atx_heading
| | | inline
| | paragraph
| | | inline
| | section <----- enter section (2) ... depth = 2
| | | atx_heading
| | | | inline
| | | paragraph
| | | | inline
| | | <----------- leave section (2) ... depth = 1
| | <-------------- leave section (1) ... depth = 0
the following stanzas should be self-explanatory now:
enter atx_heading {
print("<h");
print(depth);
print(">");
}
leave atx_heading {
print("</h");
print(depth);
print(">\n");
}
enter inline {
print(text(node));
}
but an explanation is included nonetheless:
document ... depth = 0
| section <-------- enter section (1) ... depth = 1
| | atx_heading <- enter atx_heading ... print "<h1>"
| | | inline <--- enter inline ... print ..
| | | <----------- leave atx_heading ... print "</h1>"
| | paragraph
| | | inline <--- enter inline ... print ..
| | section <----- enter section (2) ... depth = 2
| | | atx_heading enter atx_heading ... print "<h2>"
| | | | inline <- enter inline ... print ..
| | | | <-------- leave atx_heading ... print "</h2>"
| | | paragraph
| | | | inline <- enter inline ... print ..
| | | <----------- leave section (2) ... depth = 1
| | <-------------- leave section (1) ... depth = 0
the examples directory contains a complete markdown-to-html
converter, along with a few other motivating examples.
[0]: https://github.com/tree-sitter-grammars/tree-sitter-markdown
[1]: https://git.peppe.rs/cli/tree-viz
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