introducing tablespoon

3 min·87 sentences·91.95 cm·01.08.2024

tbsp (tree-based source-processing language) 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. 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):

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.

Usage

The tbsp evaluator is written in rust, use cargo to build and run:

cargo build --release
./target/release/tbsp --help

tbsp requires three inputs:

a tbsp program, referred to as "program file"
a language
an input file or some input text at stdin

You can run the interpreter like so (this program prints an overview of a rust file):

$ ./target/release/tbsp \
      -f./examples/code-overview/overview.tbsp \
      -l rust \
      src/main.rs
module
   └╴struct Cli
   └╴trait Cli
      └╴fn program
      └╴fn language
      └╴fn file
   └╴fn try_consume_stdin
   └╴fn main