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The dash AST

The dash AST itself is defined in src/nodes.h and src/parser.h

  • union node in src/nodes.h on line 146 for commands/statements node.type type tag

    node.nbinary (AND, OR, SEMI)

  • special characters and codes in parser.h on lines 40-64

    CTL* for control codes in words breaks multibyte characters/UTF-8 :(

    VS* for variable format metadata

The parser in src/parser.c is not easy to read, but is a good place to see dash ASTs being constructed.

Input sources come in a stack (to support, e.g., the source/. command). Dash has some subtle invariants around its own string allocation stack... it took quite some time to get it right in Smoosh, so don't try to "optimize" things! (See mgree/smoosh#18 for test cases.)

To get a gist for how they're used, look at evaltree at line 200 in src/eval.c. To see how the special characters and codes are used, see argstr at line 23 in src/expand.c.

OCaml bindings

The core OCaml bindings are in ocaml/dash.ml. The bindings are dynamically loaded by ctypes. (It's a longstanding TODO to make these bindings static, as it would significantly simplify the build process.)

Lines 69 through 233 are just copying the definitions from src/nodes.h.

The primary API entry point is parse_next, which returns one of a few results:

  • Done when EOF (dash returns the special node neof, not NULL!) has been reached for the current input.

  • Error when parsing failed (dash returns the special node nerr, not NULL!).

  • Null when there was no command, e.g., a blank line (dash returns NULL here).

  • Parsed n for some node, n. Note that n is a dash AST, i.e., a ctypes structure.

These nodes are dash AST nodes not yet a usable OCaml structure.

AST translation

See ocaml/ast.ml (or Smoosh's src/shim.ml for a more battle-hardened, nearly but not quite identical version of the same code) for the of_node entry point that converts dash AST nodes to OCaml structures.

parse_arg is a funny a stack machine, best thought of as a for loop with an explicit stack. There are some tricky extra bits of information to track (i.e., when tildes are possible, whether we're in an assignment).

General approach

Call Dash.initialize, then Dash.parse with the string you have. Call Ast.of_node on the resulting dash AST to get a nice OCaml structure.