tclrs

Tcl as a fusevm frontend. Tcl source is parsed and lowered to fusevm bytecode, which fusevm executes and compiles to native code through its Cranelift JIT. There is no interpreter loop and no code generator in this crate — those belong to the VM.

The reference implementation is tclsh 9.0.4. It is the specification: behavior is ported from it, not reinvented, and the test suite compares against it directly rather than against expectations written by hand.

Status

Phase 2 of 7 — scripts compile to fusevm bytecode and run. tclrs::eval executes a script and returns its value and output.

Working commands: set, puts (with -nonewline), expr, incr, if/elseif/else, while, break, continue, and command substitution of any of them.

expr covers the whole operator set of expr(n) — arithmetic with Tcl's floored integer division and remainder, integral **, numeric-preferring comparisons with string fallback, the always-string comparisons, bitwise and shift operators, short-circuit &&/||, and the ternary — over operands drawn from literals, variables, nested commands, and parenthesised subexpressions. Doubles print in Tcl's format.

Not built yet, and refused at compile time rather than approximated: proc and every other command, arrays, {*} expansion, math functions, in/ni (list support), variable and body words that are not literal, and arbitrary-precision integers — an operation that overflows i64 is an error instead of silently wrapping.

Parser

tclrs::parse implements all twelve syntax rules of Tcl(n):

Rule	Covered by
1 Commands, 3 Words	command and word splitting, line tracking
2 Evaluation	words retained in order for the compiler
4 Double quotes	quoted words with substitution
5 Argument expansion	`{*}` recorded on the word
6 Braces	nesting, literal text, backslash retention
7 Command substitution	nested scripts parsed eagerly
8 Variable substitution	`$name`, `$name(index)`, `${name}`, `${name(index)}`
9 Backslash substitution	full escape table, including the backslash-newline pre-pass
10 Comments	`#` in first-word position only
11, 12 Order and word boundaries	single pass, substitution never splits a word

Why this shape

Two properties of the grammar make compilation worthwhile:

Braces suppress substitution. A braced body is fully known at parse time, so if, while, proc bodies and braced expr expressions compile once into bytecode instead of being re-parsed on every evaluation. Words carry a braced flag for exactly this decision.

Each character is processed once. Rule 11 rules out rescanning substituted values, so the parse is single-pass and the compiler can resolve variable and command references statically where the word shape allows.

Values need no object heap. Tcl strings, integers and floats map onto fusevm::Value directly. A value produced as a number stays a number in a VM slot and only acquires a string representation when something asks for one, which is where the reference implementation spends time in hot loops.

Build

cargo build
cargo test

The differential tests run every program through both tclsh and tclrs and compare the output byte for byte. They invoke tclsh from PATH and report a skip when none is installed.

License